gh-35810: Drop support for GCC < 8.4, drop testing of `debian-buster` and `fedora-29`
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### :books: Description
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Prerequisite of:
- #34816
- #35703
- #35801
See also:
- #32074
Instead of fully dropping `debian-buster` (LTS until 2024-06) in the CI,
we replace it by `debian-buster-gcc_spkg` to verify that users on this
platform have the recourse of installing a newer GCC.
We also remove `linuxmint-19.3-gcc_8-python3.8` and change
`fedora-30-python3.8` to `fedora-30` (which builds python from spkg), to
prevent a merge conflict with
https://github.com/sagemath/sage/pull/35404
We also add `debian-trixie` (= current testing).
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35810
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik
