Backport fix for Mattermost deprecation to 18.11

What does this MR do and why?

Backport Do not fail 19.x upgrade on stale Mattermost ca... (!9594 - merged) to 18.11 (issue #10001)

Context on why is at https://gitlab.com/gitlab-org/release/tasks/-/work_items/27543#business-justification, in short - without this fix the 18.11.7 → 19.x upgrade aborts with a Mattermost error, and removing the mattermost[...] keys from gitlab.rb doesn't help, because the Mattermost salts remain in the node cache the check reads.

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 for .com
• This MR has a severity label assigned (if applicable).

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