Switch to gcc-15 in Ubuntu 26.04 Docker image #4888

Job	Run time
ci-scope	3s
pre-commit / link-check	13s
pre-commit / pre-commit	3m 26s
windows / clang-build	2m 28s
windows / msvc-build (Debug)	2m 40s
windows / msvc-build (Release)	2m 7s
mac / clang-build (Debug)	2m 2s
ubuntu / gcc-build (ubuntu-24.04, Release)	2m 23s
ubuntu / clang-build (ubuntu-24.04)	2m 54s
mac / clang-build (Release)	2m 15s
ubuntu / gcc-build (ubuntu-24.04, Debug)	1m 57s
mac / clang-test	4m 27s
ubuntu / gcc-test (ubuntu-24.04)	6m 31s
windows / clang-test	2m 53s
windows / msvc-test	4m 51s
ubuntu / clang-test (ubuntu-24.04)	6m 0s
ubuntu / clang-sanitizer-build (ubuntu-24.04)	3m 28s
windows / clang-test-extended	3m 0s
ubuntu / clang-sanitizer-test (ubuntu-24.04)	2m 27s
mac / clang-test-extended	2m 16s
windows / msvc-test-extended	1m 34s
ubuntu / clang-sanitizer-test-extended	1s
ubuntu / clang-test-extended	1s
ubuntu / gcc-test-extended	1s
ubuntu / gcc-build-codecov	1s
pages	0s
perf	0s
	59m 59s

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