Rename default branch to main

Related tasks:

• add a warning commit as a final commit for the master branch (!44 (closed))
• 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)
• edit the open MR's to have a target branch of main

Ref: mesa!10474 (merged), mesa#4501 (closed), waffle#75, drm#66 (closed)