Removed metadata from docs

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/247288

doc/install/postgresql_extensions.md

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-last_updated: 2020-09-01
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 # 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

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

doc/update/mysql_to_postgresql.md

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