alpine upgrade to 3.21

[kitshinghk-crypto]

Hi,

As alpine has the 3.21 released, it would be nice to have the upgrade in the nginxinc/nginx-unprivileged.

[alessfg]

Hi @kitshinghk-crypto, have you read the entire Readme? I'd suggest looking at this section https://github.com/nginxinc/docker-nginx-unprivileged#on-reporting-issues for more info on when the Alpine images might get updated 3.21.