gh-38008: `gcc`, `gfortran`: Update to 13.2.0; remove `debian-buster-gcc_spkg` from CI
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- Fixes #38006 @jhpalmieri
- `gcc` does not build on `debian-buster` (which we supported via
`-gcc_spkg` because it is LTS until 2024-06), so we drop this platform
now; see #32074.
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### :hourglass: Dependencies
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- Depends on #37726 (merged here to resolve merge conflict)
URL: https://github.com/sagemath/sage/pull/38008
Reported by: Matthias Köppe
Reviewer(s):
