MAINTAINERS: update ClangBuiltLinux IRC chat

Everyone has moved from Freenode to Libera so updated the channel entry
for MAINTAINERS.

Link: https://github.com/ClangBuiltLinux/linux/issues/1402
Link: https://lkml.kernel.org/r/20210818022339.3863058-1-nathan@kernel.orgSigned-off-by: NNathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -4498,7 +4498,7 @@ L:	clang-built-linux@googlegroups.com
 S:	Supported
 W:	https://clangbuiltlinux.github.io/
 B:	https://github.com/ClangBuiltLinux/linux/issues
-C:	irc://chat.freenode.net/clangbuiltlinux
+C:	irc://irc.libera.chat/clangbuiltlinux
 F:	Documentation/kbuild/llvm.rst
 F:	include/linux/compiler-clang.h
 F:	scripts/clang-tools/