Make Alpine 3.21 the default base for helper images

Stan Hurequested to merge
sh-bump-default-alpine-3.21 into main

What does this MR do?

Alpine 3.19 has reached end-of-life (https://endoflife.date/alpine). Bump the default image to Alpine 3.21.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Edited by Stan Hu

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