[cristianchiriac]

The linkcheck job in the tests workflow has been failing on main (e.g. https://github.com/abravalheri/validate-pyproject/actions/runs/36811436617) with four broken links:

docs/readme.rst:17: broken link: https://api.cirrus-ci.com/github/abravalheri/validate-pyproject.svg?branch=main (connection error)
docs/readme.rst:-1: broken link: https://cirrus-ci.com/github/abravalheri/validate-pyproject (connection error)
docs/readme.rst:-1: broken link: https://coveralls.io/r/abravalheri/validate-pyproject (403 Client Error: Forbidden)
docs/api/modules.rst:8: broken link: https://json-schema.org/understanding-json-schema/reference/string#format (Anchor 'format' not found)

• Cirrus CI shut down on 2026-06-01 (Cirrus CI to shut down Monday, June 1, 2026 #298). ci: drop Cirrus CI except for FreeBSD #302 kept .cirrus.yml for FreeBSD only, but the last Cirrus check run on main is from 2026-05-01, and cirrus-ci.com no longer answers. This PR removes .cirrus.yml and points the README "Built Status" badge at the GitHub Actions ci.yml workflow. That means FreeBSD is no longer tested anywhere. If you want it back, a vmactions/freebsd-vm job in GHA could take over; I left that out to keep this change small.
• json-schema.org renders its pages client-side (Next.js), so linkcheck can't see the #format anchor even though the link works in a browser. This adds linkcheck_anchors_ignore_for_url = [r"https://json-schema\.org/"], so the page itself is still checked but its anchors aren't.
• coveralls.io answers 403 to non-browser clients. The Coveralls badge is still used, since CI uploads coverage, so I kept it and added the URL to linkcheck_ignore.

Testing: tox -e linkcheck -- -q locally:

• on main: the four broken links above, linkcheck: FAIL;
• with this change: linkcheck: OK.