rpm: require gcc >= 13.3 regardless of gts_version

rpm: require gcc >= 13.3 regardless of gts_version

when gts_version is not set, bump the required version of gcc-c++ >= 13.3.
move this into a `%if 0%{?gts_version} == 0` block to prevent that from
applying to builds using gts, because the distro probably doesn't
provide a recent enough gcc-c++

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit f9b9bf3c6a947a483cedc00399f9d370e5158d5b)