[Doc] Upgrade 2.3 to 2.4 - Clarify GitLab server prerequisites for private repo SSH keys

[rcarre]

Note on DCO:

If the DCO action in the integration test fails, one or more of your commits are not signed off. Please click on the Details link next to the DCO action for instructions on how to resolve this.

Checklist:

• [ ] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• [ ] The title of the PR states what changed and the related issues number (used for the release note).
• [ ] I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• [ ] I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• [x] Does this PR require documentation updates?
• [x] I've updated documentation as required by this PR.
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[codecov[bot]]

Codecov Report

Merging #10135 (8795ec0) into master (506bd3b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10135   +/-   ##
=======================================
  Coverage   45.78%   45.78%           
=======================================
  Files         233      233           
  Lines       28383    28383           
=======================================
  Hits        12995    12995           
  Misses      13614    13614           
  Partials     1774     1774           

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[crenshaw-dev]

@rcarre looks like rendering in readthedocs is a little off. Can you check here? https://argo-cd--10135.org.readthedocs.build/en/10135/operator-manual/upgrading/2.3-2.4/

You can test locally with make servce-docs-local.