Prepare patch release 2.8.4

[robinmolen]

Part of #5055

Changes

• Updated changelog
• Bumped version

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.56%. Comparing base (fc3bacd) to head (261b23d).

Additional details and impacted files

@@              Coverage Diff              @@
##           stable/2.8.x    #5056   +/-   ##
=============================================
  Coverage         96.56%   96.56%           
=============================================
  Files               746      746           
  Lines             25332    25332           
  Branches           3346     3346           
=============================================
  Hits              24463    24463           
  Misses              604      604           
  Partials            265      265           

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[sergei-maertens]

There will be more changelog entries of course :)

[sergei-maertens]

Bugfix releases should basically have "Fixed ..." for every changelog entry. Additional CI fixes etc. can be omitted, it's not useful information to people that want to know which bugs are fixed (because they want to know if their problems have been solved).

Suggested change

	* [:backend:`5035`] Removed non-existing data shown in components variables and
	legacy Objects API submission data.
	* Fixed broken links in redis documentation.
	* Pinned Github actions runners to Ubuntu version 22.04.
	* Upgraded the Django framework to version 4.2.18.
	* Updated eHerkenning documentation.
	* [:backend:`5035`] Fixed the legay Objects API registration including fields from other steps in the submitted data.
	* Applied the latest Django security patches.