Remove documentation on compiling OpenSSH for CentOS 6

CentOS 6 is no longer supported as of GitLab 13.6:
https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4866

We can drop these instructions also because most users aren't compiling
a custom version of OpenSSH in any case.

doc/administration/operations/fast_ssh_key_lookup.md

@@ -26,11 +26,8 @@ indexed lookup in the GitLab database. This page describes how to enable the fas
 lookup of authorized SSH keys.
 
 WARNING:
-OpenSSH version 6.9+ is required because
-`AuthorizedKeysCommand` must be able to accept a fingerprint. These
-instructions break installations that use older versions of OpenSSH, such as
-those included with CentOS 6 as of September 2017. If you want to use this
-feature for CentOS 6, follow [the instructions on how to build and install a custom OpenSSH package](#compiling-a-custom-version-of-openssh-for-centos-6) before continuing.
+OpenSSH version 6.9+ is required because `AuthorizedKeysCommand` must be
+able to accept a fingerprint. Check the version of OpenSSH on your server.
 
 ## Fast lookup is required for Geo **(PREMIUM)**
 
@@ -170,101 +167,6 @@ To switch from OpenSSH to `gitlab-sshd`:
 
 1. Perform a Helm upgrade.
 
-## Compiling a custom version of OpenSSH for CentOS 6
-
-Building a custom version of OpenSSH is not necessary for Ubuntu 16.04 users,
-since Ubuntu 16.04 ships with OpenSSH 7.2.
-
-It is also unnecessary for CentOS 7.4 users, as that version ships with
-OpenSSH 7.4. If you are using CentOS 7.0 - 7.3, we strongly recommend that you
-upgrade to CentOS 7.4 instead of following this procedure. This should be as
-simple as running `yum update`.
-
-CentOS 6 users must build their own OpenSSH package to enable SSH lookups via
-the database. The following instructions can be used to build OpenSSH 7.5:
-
-1. First, download the package and install the required packages:
-
-   ```shell
-   sudo su -
-   cd /tmp
-   curl --remote-name "https://cdn.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-7.5p1.tar.gz"
-   tar xzvf openssh-7.5p1.tar.gz
-   yum install rpm-build gcc make wget openssl-devel krb5-devel pam-devel libX11-devel xmkmf libXt-devel
-   ```
-
-1. Prepare the build by copying files to the right place:
-
-   ```shell
-   mkdir -p /root/rpmbuild/{SOURCES,SPECS}
-   cp ./openssh-7.5p1/contrib/redhat/openssh.spec /root/rpmbuild/SPECS/
-   cp openssh-7.5p1.tar.gz /root/rpmbuild/SOURCES/
-   cd /root/rpmbuild/SPECS
-   ```
-
-1. Next, set the spec settings properly:
-
-   ```shell
-   sed -i -e "s/%define no_gnome_askpass 0/%define no_gnome_askpass 1/g" openssh.spec
-   sed -i -e "s/%define no_x11_askpass 0/%define no_x11_askpass 1/g" openssh.spec
-   sed -i -e "s/BuildPreReq/BuildRequires/g" openssh.spec
-   ```
-
-1. Build the RPMs:
-
-   ```shell
-   rpmbuild -bb openssh.spec
-   ```
-
-1. Ensure the RPMs were built:
-
-   ```shell
-   ls -al /root/rpmbuild/RPMS/x86_64/
-   ```
-
-   You should see something as the following:
-
-   ```plaintext
-   total 1324
-   drwxr-xr-x. 2 root root   4096 Jun 20 19:37 .
-   drwxr-xr-x. 3 root root     19 Jun 20 19:37 ..
-   -rw-r--r--. 1 root root 470828 Jun 20 19:37 openssh-7.5p1-1.x86_64.rpm
-   -rw-r--r--. 1 root root 490716 Jun 20 19:37 openssh-clients-7.5p1-1.x86_64.rpm
-   -rw-r--r--. 1 root root  17020 Jun 20 19:37 openssh-debuginfo-7.5p1-1.x86_64.rpm
-   -rw-r--r--. 1 root root 367516 Jun 20 19:37 openssh-server-7.5p1-1.x86_64.rpm
-   ```
-
-1. Install the packages. OpenSSH packages replace `/etc/pam.d/sshd`
-   with their own versions, which may prevent users from logging in, so be sure
-   that the file is backed up and restored after installation:
-
-   ```shell
-   timestamp=$(date +%s)
-   cp /etc/pam.d/sshd pam-ssh-conf-$timestamp
-   rpm -Uvh /root/rpmbuild/RPMS/x86_64/*.rpm
-   yes | cp pam-ssh-conf-$timestamp /etc/pam.d/sshd
-   ```
-
-1. Verify the installed version. In another window, attempt to sign in to the
-   server:
-
-   ```shell
-   ssh -v <your-centos-machine>
-   ```
-
-   You should see a line that reads: "debug1: Remote protocol version 2.0, remote software version OpenSSH_7.5"
-
-   If not, you may need to restart `sshd` (for example, `systemctl restart sshd.service`).
-
-1. *IMPORTANT!* Open a new SSH session to your server before exiting to make
-   sure everything is working! If you need to downgrade, simple install the
-   older package:
-
-   ```shell
-   # Only run this if you run into a problem logging in
-   yum downgrade openssh-server openssh openssh-clients
-   ```
-
 ## SELinux support and limitations
 
 GitLab supports `authorized_keys` database lookups with [SELinux](https://en.wikipedia.org/wiki/Security-Enhanced_Linux).

doc/administration/operations/ssh_certificates.md

@@ -19,10 +19,8 @@ In such setups some external automated process is needed to constantly
 upload the new keys to GitLab.
 
 WARNING:
-OpenSSH version 6.9+ is required because that version
-introduced the `AuthorizedPrincipalsCommand` configuration option. If
-using CentOS 6, you can [follow these instructions](fast_ssh_key_lookup.md#compiling-a-custom-version-of-openssh-for-centos-6)
-to compile an up-to-date version.
+OpenSSH version 6.9+ is required because `AuthorizedKeysCommand` must be
+able to accept a fingerprint. Check the version of OpenSSH on your server.
 
 ## Why use OpenSSH certificates?