docs: Deprecated API elements scheduled for removal in APIv5

Several groups have API elements that were deprecated as long ago as GitLab 12.3. The groups hope to remove those elements in v5 of our API, instead of a specific milestone. From a documentation perspective, this lack of a milestone creates a problem:

• our deprecations / removals page focuses on GitLab release versions
• we don't know when (or if) v5 will happen
• we don't have another method for announcing these changes to API fields

Our deprecation / removal announcement process requires us to pick a milestone, and that's leading to items listed as being removed in %16.0 when they're really dependent on APIv5. From a Slack discussion this morning with @sselhorn and @dianalogan, it's time to bring this up in a separate issue.

awareness: @sselhorn and @dianalogan from our discussion this morning. @phikai and @tlinz as PMs for groups in devopscreate that are affected.

Assigning to Farnoosh and Brian for discussion.

Related issues

• Aligning REST to GitLab version instead (#387485 - closed) will there be an APIv5 at all?
• Start v5 API (#216456 - closed)

Merge requests

• Related to Remove milestone for merged_by deprecation (!99360 - merged) - Amy outlined the problem and a possible solution in 15.6 in this comment
• Brought up in #30858 (comment 1226512167) from groupcode review
• Deprecate approvers and approval_group fields (!108621 - merged) in 15.8
• Deprecate single merge changes endpoint (!105706 - merged) in 15.7
• Implement new diffs REST API endpoint (!104561 - merged) in 15.7
• REST API: add field merge_user to MR response (!77658 - merged) in 14.8