Upgrade clang + CUDA CI pipeline to use CUDA 12

[msimberg]

This should allow using C++20 again for both the spec and the build of pika. This also allows removing the +allow-unsupported-compilers variant.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.01% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (11dfb31)	18223	13730	75.34%
Head commit (decadb1)	18223 (+0)	13732 (+2)	75.36% (+0.01%)

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 (#1366)	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]

Well, now clang is old enough for CUDA to support, but CUDA is too new for clang to support. This may be causing errors. I'll come back to this once other CI PRs have settled a bit.