better kernel version sorting

should now detect that 6.12.16 is newer than 6.6.74 for example

sau/gentoo.py

@@ -419,7 +419,8 @@ def pkg_upgrade():
             for sysfile in ['config', 'initramfs', 'System.map', 'vmlinuz', 'kernel']:
                 match = sorted(
                         [f for f in files if f.startswith(f'{sysfile}-')],
-                        reverse=True)
+                        reverse=True,
+                        key=lambda x: tuple(map(int, x.split('-')[1].split('.')[:3])))
                 for f in match[keep_kernels:]:
                     log.debug(f"Removing old kernel file {f}")
                     os.remove(os.path.join(root, f))