Update mainline to Alpine 3.21

[jnoordsij]

Proposed changes

Updates the mainline image to use Alpine 3.21 as default version. See also https://wiki.alpinelinux.org/wiki/Release_Notes_for_Alpine_3.21.0.

This first requires the Alpine 3.21 base image to appear; see also docker-library/official-images#18024.

Note: given that this requires built binaries for the new Alpine version and won't take any effect until an actual new release of nginx itself, this PR is intentionally marked as draft, so it can function both as a heads-up about the new release (in anticipation of new mainline release that will probably appear soon?) and a place that allows for subscription to any potential updates. It can then be merged later at any convenient time when everything is ready. However, if it is still preferable to close this in the meantime, feel free to do so.

Modelled after #894.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the CONTRIBUTING document
• I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation