Updating Alpine deploy image version to be in line with build image version

[yamatt]

Updates deploy version of Alpine from 3.18 to 3.21

• 3.18 is no longer supported
• 3.21 is the same as the build version

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[aptalca]

Might as well rebase to alpine 3.22 now that it's out

[yamatt]

@aptalca I agree, although I didn't know which direction to go. Aline it to what is already in use, or go the whole way?

[aptalca]

When I realized your PR was 2 weeks old and no CLA, I assumed you changed your mind or perhaps didn't want to sign their CLA (would be understandable as it requires a Google acct) so I opened a new PR with another change:
#3705

If you want to integrate those here and sign the CLA, let me know and I'll close the other PR

[yamatt]

I did sign it, I think - but I wasn't sure what the next steps were.

Happy if it's been moved in to another PR, as long as Alpine is on a supported version.