Switch to gcc-15 in Ubuntu 26.04 Docker image (#793) #4890

Job	Run time
ci-scope	2s
pre-commit / pre-commit	2m 19s
pre-commit / link-check	15s
ubuntu / clang-build (ubuntu-24.04)	2m 42s
ubuntu / gcc-build (ubuntu-24.04, Release)	2m 35s
mac / clang-build (Release)	2m 9s
windows / clang-build	5m 11s
ubuntu / gcc-build (ubuntu-24.04, Debug)	1m 57s
mac / clang-build (Debug)	2m 19s
windows / msvc-build (Release)	1m 54s
windows / msvc-build (Debug)	3m 6s
mac / clang-test	4m 32s
ubuntu / gcc-test (ubuntu-24.04)	8m 35s
ubuntu / clang-sanitizer-build (ubuntu-24.04)	3m 12s
ubuntu / clang-test (ubuntu-24.04)	8m 42s
windows / msvc-test	4m 44s
windows / clang-test	3m 21s
ubuntu / clang-sanitizer-test (ubuntu-24.04)	2m 58s
mac / clang-test-extended	3m 14s
windows / msvc-test-extended	2m 52s
windows / clang-test-extended	2m 56s
ubuntu / clang-sanitizer-test-extended (ubuntu-24.04)	2m 26s
ubuntu / gcc-test-extended (ubuntu-24.04)	4m 50s
ubuntu / clang-test-extended (ubuntu-24.04)	4m 39s
ubuntu / gcc-build-codecov	7m 24s
perf / macos-clang-build-perf-stats	1m 41s
perf / ubuntu-gcc-build-perf-stats	44s
pages / build-scoreboard	18s
pages / build-doxygen-xml	20s
pages / build-sphinx	39s
pages / deploy-pages	0s
	1h 32m 36s

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