Switch to rust:*-slim-bullseye as base

[urschrei]

This is an attempt to reduce our æons-long image rebuilds (and geo and proj's CI too ofc). It drops the base image size from 1.33 GB to 754 MB uncompressed, and the final libproj-builder image from 2.00 GB to 1.57 GB uncompressed (~21% reduction).

Dependent image size reduction:

geo-ci: from ~1.58 GB --> 1.15 GB
proj-ci: from ~2.02 GB --> 1.59 GB
proj-ci-without-system-proj: from ~2.00 GB --> 1.57 GB

• I agree to follow the project's code of conduct.
• I added an entry to the project's change log file if knowledge of this change could be valuable to users.
  • Usually called CHANGES.md or CHANGELOG.md
  • Prefix changelog entries for breaking changes with "BREAKING: "

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