matrix.py: bump LLVM version from 20 to 21 (#409)

Signed-off-by: Ihor Solodrai <[email protected]>

Author: theihor

.github/scripts/matrix.py

@@ -19,7 +19,7 @@
 DEFAULT_SELF_HOSTED_RUNNER_TAGS: Final[List[str]] = ["self-hosted", "docker-noble-main"]
 DEFAULT_GITHUB_HOSTED_RUNNER: Final[str] = "ubuntu-24.04"
 DEFAULT_GCC_VERSION: Final[int] = 15
-DEFAULT_LLVM_VERSION: Final[int] = 20
+DEFAULT_LLVM_VERSION: Final[int] = 21
 
 RUNNERS_BUSY_THRESHOLD: Final[float] = 0.8