Deprecate 'omnibus_gitconfig'

This commit adds a deprecation for 'omnibus_gitconfig'. Git configuration
should be directly added under Gitaly's configuration as directed in the
linked migration instructions. Configuring Git on a system or user level
will no longer be supported through Omnibus.

Related https://gitlab.com/gitlab-org/gitaly/-/issues/5132

Changelog: deprecated

Author: Sami Hiltunen

files/gitlab-cookbooks/package/libraries/deprecations.rb

@@ -355,6 +355,12 @@ def list(existing_config = nil)
             deprecation: '16.9',
             removal: '17.0',
             note: "Starting with GitLab 17.0, `sidekiq['max_concurrency']` will be removed. Please follow https://docs.gitlab.com/ee/administration/sidekiq/extra_sidekiq_processes.html#manage-thread-counts-explicitly to use `sidekiq['concurrency']` instead."
+          },
+          {
+            config_keys: %w(gitlab omnibus_gitconfig),
+            deprecation: '16.10',
+            removal: '17.0',
+            note: "`omnibus_gitconfig` will be removed in GitLab 17.0. For details and migration instructions, please see: https://docs.gitlab.com/ee/update/versions/gitlab_16_changes.html#gitlabomnibus_gitconfig-deprecation"
           }
         ]