fix: update default LLVM to v20 + GCC 15 #2084

Job	Run time
Build (ubuntu-22.04)	1m 11s
Build-Executable-macos-13	51s
Build-Executable-macos-14	33s
Build-Executable-ubuntu-22.04-arm	47s
Test-ubuntu-24.04-arm	9m 10s
Test-windows-2019	9m 44s
Build-Executable-ubuntu-22.04	45s
Build-Executable-windows-2019	1m 16s
Test-windows-2022	7m 15s
Test-ubuntu-22.04	9m 59s
Test-ubuntu-22.04-arm	9m 57s
Test-ubuntu-24.04	9m 59s
Test-macos-15	3m 53s
Test-macos-14	5m 1s
Test-macos-13	6m 1s
Docker-setup-cpp-fedora-linux/amd64-40	6m 39s
Docker-setup-cpp-ubuntu-linux/amd64-22.04	5m 40s
Docker-setup-cpp-ubuntu-linux/arm64-22.04	7m 5s
Docker-setup-cpp-ubuntu-linux/arm64-20.04	6m 44s
Docker-setup-cpp-alpine-linux/arm64-3.21	3m 34s
Docker-setup-cpp-ubuntu-linux/amd64-20.04	6m 0s
Docker-setup-cpp-arch-linux/amd64-base	2m 44s
Docker-setup-cpp-ubuntu-linux/arm64-24.04	6m 47s
Docker-setup-cpp-ubuntu-linux/amd64-24.04	5m 26s
Docker-setup-cpp-alpine-linux/amd64-3.21	3m 10s
Docker-Manifest	0s
Release	1s
	2h 10m 12s

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