Document transition from ANSI-C towards C-99

Document transition from ANSI-C towards C-99

The existing NOTES-ANSI.md are renamed to NOTES-C99.md and updated
accordingly. INSTALL.md lists C-99 compiler instead of ANSI-C now.
Also moving from ANSI-C to C-99 warrants updates to NEWS.md and
CHANGES.md.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/27751)