Switch base image to buster/buster+bullseye for next major Perl release

[zakame]

cf docker-library/official-images#8405 docker/docker-ce-packaging#476 (comment)

I'll echo my reply to the linked official-images PR above:

Speaking on behalf of library/perl, we'll likely continue with using stretch+buster for the current supported perl:5.32 (latest) and perl:5.30 tags, in line with perlpolicy, along with the current best-effort-supported perl:5.28. Older Perl releases (5.26 and below) are no longer supported and are also no longer rebuilt here anyway (but we will retain their existing Dockerfiles in https://github.com/Perl/docker-perl should anyone need them.)

We will switch to buster+bullseye (or buster only if bullseye hasn't been released yet,) for whatever comes next, be it perl:5.34 or perl:7.

[zakame]

Soonest we can do this now seems to be this May: https://www.nntp.perl.org/group/perl.perl5.porters/2021/01/msg258879.html

[zakame]

This is now available: https://hub.docker.com/_/perl?tab=tags&page=1&ordering=last_updated&name=5.34