Draft: Add maintenance mode

Why is this change being made?

As deprecations hint on a future removal, marking a feature where removal is not yet planned as deprecated will likely create confusion to our users. This MR introduces "maintenance mode" as a state into the product lifecycle.

Features that would qualify immediately as being in maintenance mode are:

• Only/Except CI syntax
• Certificate based cluster integration

As features moved to maintenance mode should receive a release post item, the recommendation is to initially use the release post itemdeprecation label and machinery. We can extend the scripts later together with adding ~"release post item::maintenance" label.

As a result, this change would primarily enable improved communication towards our users through the GitLab docs and UI.

If this proposal is accepted, the GitLab docs should be updated too.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
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• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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