Rename the default branch from master to main

[ebeahan]

The ECS team will follow these steps to ensure a smooth transition when renaming the default branch from master to main:

• Add temporary workflow to sync master to main branch #1644 Synchronize the new main branch using a GitHub Action workflow. Example
• Add main branch to the ecs repo docs build docs#2266 Update the docs build config to point to main. Example
• Rename the default branch as explained in the GitHub Docs.
• Update any branch protection rules for master to now apply to main.
• Remove GitHub Action workflow synchronizing master to main.