Remove mentions of Cirrus CI - #3625

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, github/codeql-action/analyze@v3, github/codeql-action/autobuild@v3, github/codeql-action/init@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Error retrieving diff master...peterebden:peter/rm-cirrus: Server Error: Sorry, this diff is temporarily unavailable due to heavy server load.: {"resource":"Comparison","field":"diff","code":"not_available"} - https://docs.github.com/rest/commits/commits#compare-two-commits

CodeQL Action v3 will be deprecated in December 2026. Please update all occurrences of the CodeQL Action in your workflow files to v4. For more information, see https://github.blog/changelog/2025-10-28-upcoming-deprecation-of-codeql-action-v3/