Merge branch 'selhorn-remove-metadata' into 'master'

Removed metadata from docs

See merge request gitlab-org/gitlab!41940

doc/install/postgresql_extensions.md

----
-last_updated: 2020-09-01
----
-
 # Managing PostgreSQL extensions
 
 This guide documents how to manage PostgreSQL extensions for installations with an external

doc/topics/git/lfs/migrate_to_git_lfs.md

 ---
-type: tutorial, concepts
 description: "How to migrate an existing Git repository to Git LFS with BFG."
-last_updated: 2019-07-11
 ---
 
 # Migrate a Git repo into Git LFS with BFG

doc/update/mysql_to_postgresql.md

----
-last_updated: 2019-06-18
----
-
 # Migrating from MySQL to PostgreSQL
 
 This guide documents how to take a working GitLab instance that uses MySQL and