add alpine 3.21 to the build matrix

[pdabelf5]

Proposed changes

Add support for Alpine 3.21

Checklist

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• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

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