Remove iron workflows and update readme (backport #612)

[mergify]

Due after the next patch release on November 15th.

---

This is an automatic backport of pull request #612 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 7b7f70c has failed:

On branch mergify/bp/humble/pr-612
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 7b7f70c.
  (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:   .github/mergify.yml
	deleted:    .github/workflows/iron-binary-build.yml
	deleted:    .github/workflows/iron-check-docs.yml
	deleted:    .github/workflows/iron-docker-build.yaml
	deleted:    .github/workflows/iron-pre-commit.yml
	deleted:    .github/workflows/iron-semi-binary-build.yml
	modified:   README.md

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   .github/dependabot.yml
	deleted by them: ros2_control_demos-not-released.foxy.repos
	deleted by them: ros2_control_demos.foxy.repos

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