Bump alpine default tag to 3.21 (#11157)
## Summary
Alpine 3.21 has been released for a few months and it's now being used
officially under `alpine` based [python
images](https://hub.docker.com/_/python), hence our python-alpine based
images has been using 3.21 since uv 0.5.8 under the hood.
This could arguably be `breaking` as we're dropping alpine3.20 top-level
tag, so it could be a good candidate for 0.6.0.
Alternatively, we can keep support for 3.20 and make this non-breaking
by simply repointing alpine to now be 3.21 and keeping the 3.20 tag
around.
