Update crontab and git_update_version.py

2021-04-20  Jakub Jelinek  <jakub@redhat.com>

maintainer-scripts/
	* crontab: Snapshots from trunk are now GCC 12 related.
	Add GCC 11 snapshots from the respective branch.
contrib/
	* gcc-changelog/git_update_version.py (active_refs): Add
	releases/gcc-11.