backport: add mattermost and spamcheck deprecation entries to 18.11

What does this MR do and why?

Backports the mattermost and spamcheck deprecation/removal entries from master (added in the 19.0 package via !9408 (merged)) to the 18-11-stable branch.

Without these entries, gitlab-ctl check-config --version 19.0.x silently passes on an 18.11 installation even when mattermost[...] or spamcheck[...] keys are present in gitlab.rb. This is because check-config is a pre-upgrade tool that runs against the currently installed package — so the deprecation entries must exist in 18.11 for the check to fire before the user upgrades to 19.0.

Entries added:

• mattermost[...] — deprecated and removed in 19.0
• mattermost_external_url — deprecated in 19.0, removal in 20.0
• spamcheck[...] — deprecated and removed in 19.0

Related discussion: !9492 (comment 3489735158) Original master MR: !9492

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) - n/a, deprecation change that .com doesn't use
• This MR has a severity label assigned (if applicable).

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