Enable workhorse_use_sidechannel by default

This changes the workhorse_use_sidechannel feature flag to be on by
default. This is a performance improvement for Git fetch traffic, see
https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/463.

For self-managed installations that have configured a gRPC proxy to
sit between Workhorse and Gitaly, this will break things. The solution
for that is to remove the proxy or change it to a TCP proxy. Having a
proxy between Workhorse and Gitaly is not a recommended configuration
but we cannot rule out that someone chose to do this. Also see
https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1301.

This commit intentionally leaves the feature flag around so that in
case someone runs into this problem, they have a temporary workaround
(disabling the feature flag) besides the real solution (not using a
gRPC proxy).

Changelog: performance

config/feature_flags/development/workhorse_use_sidechannel.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1
 milestone: '14.4'
 type: development
 group: 'group::scalability'
-default_enabled: false
+default_enabled: true

doc/update/index.md

@@ -305,10 +305,14 @@ and [Helm Chart deployments](https://docs.gitlab.com/charts/). They come with ap
 
 ### 14.5.0
 
-When `make` is run, Gitaly builds are now created in `_build/bin` and no longer in the root directory of the source directory. If you
+- When `make` is run, Gitaly builds are now created in `_build/bin` and no longer in the root directory of the source directory. If you
 are using a source install, update paths to these binaries in your [systemd unit files](upgrading_from_source.md#configure-systemd-units)
 or [init scripts](upgrading_from_source.md#configure-sysv-init-script) by [following the documentation](upgrading_from_source.md).
 
+- Connections between Workhorse and Gitaly use the Gitaly `backchannel` protocol by default. If you deployed a gRPC proxy between Workhorse and Gitaly,
+  Workhorse can no longer connect. As a workaround, [disable the temporary `workhorse_use_sidechannel`](../administration/feature_flags.md#enable-or-disable-the-feature)
+  feature flag. If you need a proxy between Workhorse and Gitaly, use a TCP proxy.
+
 ### 14.4.0
 
 Git 2.33.x and later is required. We recommend you use the