Make GCC 15 CI pipeline use C++26

[msimberg]

Update GCC 14 pipeline to use C++23 so that at least one GCC configuration uses C++23.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.05% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7fc53f0)	18193	13695	75.28%
Head commit (333356b)	18193 (+0)	13705 (+10)	75.33% (+0.05%)

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 (#1447)	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]

Not updating stdexec commit because of #1448. The task_size --method=bulk benchmark runs very slowly because it doesn't seem to be parallelized.

[msimberg]

Putting this PR on hold. It'd be nice to have this build use stdexec as that's what we're targeting as the default option. However, an older version of stdexec (25.03 tag) doesn't build with GCC 15. A newer version of stdexec (main as of today) has removed algorithm customizations through tag_invoke. Waiting until we update the algorithm customizations.