fix: update default LLVM to v20 + GCC 15 #2083

Job	Run time
Build (ubuntu-22.04)	1m 12s
Build-Executable-ubuntu-22.04-arm	33s
Build-Executable-macos-13	1m 3s
Build-Executable-windows-2019	1m 27s
Test-windows-2022	8m 56s
Test-ubuntu-24.04-arm	11m 5s
Test-ubuntu-22.04-arm	10m 54s
Test-macos-15	4m 43s
Test-ubuntu-22.04	9m 48s
Test-macos-14	5m 1s
Test-ubuntu-24.04	11m 6s
Docker-setup-cpp-ubuntu-linux/amd64-24.04	4m 55s
Test-windows-2019	10m 50s
Build-Executable-macos-14	46s
Docker-setup-cpp-ubuntu-linux/amd64-20.04	5m 38s
Test-macos-13	6m 10s
Build-Executable-ubuntu-22.04	25s
Docker-setup-cpp-fedora-linux/amd64-40	5m 43s
Docker-setup-cpp-ubuntu-linux/amd64-22.04	5m 37s
Docker-setup-cpp-arch-linux/amd64-base	2m 39s
Docker-setup-cpp-ubuntu-linux/arm64-24.04	6m 26s
Docker-setup-cpp-alpine-linux/amd64-3.21	2m 56s
Docker-setup-cpp-ubuntu-linux/arm64-20.04	6m 28s
Docker-setup-cpp-ubuntu-linux/arm64-22.04	6m 47s
Docker-setup-cpp-alpine-linux/arm64-3.21	3m 40s
Docker-Manifest	0s
Release	1s
	2h 14m 49s

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