Use GCC 13 branch, Linux 6.3 in build-many-glibcs.py

This patch updates build-many-glibcs.py to use Linux 6.3 and GCC 13
branch by default.

Tested with build-many-glibcs.py (host-libraries, compilers and glibc
builds).

scripts/build-many-glibcs.py

@@ -792,10 +792,10 @@ class Context(object):
     def checkout(self, versions):
         """Check out the desired component versions."""
         default_versions = {'binutils': 'vcs-2.40',
-                            'gcc': 'vcs-12',
+                            'gcc': 'vcs-13',
                             'glibc': 'vcs-mainline',
                             'gmp': '6.2.1',
-                            'linux': '6.2',
+                            'linux': '6.3',
                             'mpc': '1.3.1',
                             'mpfr': '4.2.0',
                             'mig': 'vcs-mainline',