[19.2] Backport fixes for Mattermost deprecation

What does this MR do and why?

MR backports Do not fail 19.x upgrade on stale Mattermost ca... (!9594 - merged) and Fix accidental Mattermost enable on master + ge... (!9652 - merged) to 19.2

Why: If customer has mattermost['enable'] = false in gitlab.rb in 18.11.x - it won't be flagged by upgrade check gitlab-ctl check-config --version=19.2. However 19.x code still has old deprecation check without above fixes - older deprecation code flags this field and fails upgrade FATAL: RuntimeError: Removed configurations found in gitlab.rb. Aborting reconfigure. as reported in #10001 (comment 3669602836). This MR backports existing fixes to 19.x stable branch to align on the upgrade process experience.

Validation

Images built in current MR has been validated using scripts to trigger upgrade from 18.11.9 to current image from MR (Docker ARM image locally and Ubuntu AMD image in VM in GCP). See results in gitlab$6030720 and the scripts with readme attached too.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
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