Change default branch to main

Following up a similar change in crucible#5 (closed), let's change the default piglit branch to main.

• add a warning commit as a final commit for the master branch
  • !496 (merged)
• push the main branch as a duplicate of master
• set the default branch to main
• lock the old master branch (so the last commit will be a warning commit)
• configure mesa GitLab CI to use the main branch from piglit
  • Based on git grep 'git[ ].*piglit' .gitlab-ci, I think mesa CI uses the 'default' piglit branch
• make sure Intel CI is using the main branch
  • Mesa_CI/mesa_jenkins!1695 (merged)
• edit the open MR's to have a target branch of main

Ref: crucible@db7cc39e
Ref: Mesa_CI/mesa_jenkins!1694 (merged)
Ref: https://gitlab.com/jljusten/src-fdo/-/blob/master/gitlab-retarget-mr.py