After internal discussions we'll now proceed to move over to the main branch.

To-do:

• Update references to point to main branches for other repositories where applicable: chore: update reference of master branch to main for other repositories #7154
• Update default branch to main branch
• Update references to point to main branch for this repository: chore: update to reference main branch after default branch migration #7166
• Update Kokoro jobs: test: update Kokoro to use main branch #7167, cl/412994041