posix: signals: deprecate CONFIG_POSIX_LIMITS_RTSIG_MAX

There were two Kconfig constants defined for (effectively) the
same thing in Zephyr. Namely, RTSIG_MAX - the highest valued
real-time signal.

Deprecate CONFIG_POSIX_LIMITS_RTSIG_MAX in favour of
CONFIG_POSIX_RTSIG_MAX since the latter more closely matches
convention.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>