Switch to gcc-15 in Ubuntu 26.04 Docker image #4889

Job	Run time
pre-commit / pre-commit	2m 23s
pre-commit / link-check	14s
ci-scope	7s
ubuntu / clang-build (ubuntu-24.04)	2m 54s
ubuntu / gcc-build (ubuntu-24.04, Debug)	1m 52s
windows / clang-build	3m 36s
windows / msvc-build (Release)	1m 47s
mac / clang-build (Debug)	2m 18s
ubuntu / gcc-build (ubuntu-24.04, Release)	2m 2s
windows / msvc-build (Debug)	3m 13s
mac / clang-build (Release)	2m 51s
ubuntu / gcc-test (ubuntu-24.04)	8m 43s
mac / clang-test	5m 9s
ubuntu / clang-test (ubuntu-24.04)	8m 32s
ubuntu / clang-sanitizer-build (ubuntu-24.04)	3m 8s
windows / msvc-test	5m 46s
windows / clang-test	4m 3s
ubuntu / clang-sanitizer-test (ubuntu-24.04)	2m 55s
windows / clang-test-extended	2m 56s
mac / clang-test-extended	2m 39s
ubuntu / clang-sanitizer-test-extended (ubuntu-24.04)	2m 27s
windows / msvc-test-extended	2m 47s
ubuntu / gcc-test-extended (ubuntu-24.04)	4m 39s
ubuntu / clang-test-extended (ubuntu-24.04)	4m 37s
ubuntu / gcc-build-codecov	7m 24s
perf / ubuntu-gcc-build-perf-stats	37s
perf / macos-clang-build-perf-stats	1m 14s
pages / build-scoreboard	19s
pages / build-doxygen-xml	21s
pages / build-sphinx	36s
pages / deploy-pages	0s
	1h 32m 9s

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