Remove Iron workflows and from README (backport #230)

[mergify]

Since Iron is now EOL, we will not add any further changes to Iron.

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This is an automatic backport of pull request #230 done by Mergify.

[mergify]

Cherry-pick of 41ee92b has failed:

On branch mergify/bp/humble/pr-230
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 41ee92b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   README.md

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/mergify.yml
	both modified:   ci_status.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally