Use Linux 6.8 in build-many-glibcs.py

This patch makes build-many-glibcs.py use Linux 6.8.

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

scripts/build-many-glibcs.py

@@ -832,7 +832,7 @@ class Context(object):
                             'gcc': 'vcs-13',
                             'glibc': 'vcs-mainline',
                             'gmp': '6.3.0',
-                            'linux': '6.7',
+                            'linux': '6.8',
                             'mpc': '1.3.1',
                             'mpfr': '4.2.1',
                             'mig': 'vcs-mainline',