Update base image used in CSCS CI to Ubuntu 24.04

[msimberg]

No description provided.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.02% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (f182d67)	18212	13736	75.42%
Head commit (87c0d3d)	18212 (+0)	13732 (-4)	75.40% (-0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1317)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[msimberg]

I think I will need to modify this so that one can choose the base GCC/libstdc++ version when using clang. Not all lisbtdc++ versions are compatible with all versions of clang. Currently we just assume that the default system GCC is good enough for all the clang configurations.