diagnostics: mark the JSON output format as deprecated

diagnostics: mark the JSON output format as deprecated

The bulk of the documentation for -fdiagnostics-format= is taken up
by a description of the "json" format added in r9-4156-g478dd60ddcf177.

I don't plan to add any extra features to the "json" format; all my
future work on machine-readable GCC diagnostics is likely to be on the
SARIF output format (https://gcc.gnu.org/wiki/SARIF).

Hence users seeking machine-readable output from GCC should use SARIF.

This patch removes the long documentation of the format and describes it
as deprecated.

gcc/ChangeLog:
	* doc/invoke.texi (fdiagnostics-format): Describe "json" et al as
	deprecated, and remove the long description of the output format.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>