c: Do not document C23 support as experimental and incomplete

c: Do not document C23 support as experimental and incomplete

Since C23 support is substantially feature-complete, update
documentation to no longer refer to it as experimental and incomplete.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/
	* doc/cpp.texi (__STDC_VERSION__): Do not refer to C23 support as
	experimental.
	* doc/invoke.texi (std=c23, std=gnu23): Do not document as
	experimental and incomplete.
	* doc/standards.texi: Do not refer to C23 support as experimental
	and incomplete.

gcc/c-family/
	* c.opt (std=c23, std=gnu23, std=iso9899:2024): Do not mark as
	experimental and incomplete.