Remove obsolete master→main migration notes from README

[Copilot]

Pull Request Template: Add a Conference or Workshop

Thank you for contributing! Most PRs are to add a new conference or workshop. The following are to help ensure you've added everything correctly:

Conference/Workshop Details

• Name, Location, Date(s), Website URL and Status are required
• X / Twitter is Optional
• Location should be City + Country or Online
• Status can include information about registration, pricing, calls for proposals and can include links to all the above.

Checklist

• I have added the conference/workshop to the correct YAML file (_data/current.yml or _data/past.yml)
• The entry includes name, location, date(s), url and accurate status
• The entry is in chronological order (soonest to furtherest away)
• If there are any special characters in the name field (: or ; or '), the name must be in quotes (")
• I have checked for duplicates to avoid listing the same event twice
• Build runs successfully

Additional context

This PR is a README cleanup: it removes outdated references to the historical branch rename from master to main, including both the callout and follow-up note.

• Documentation update
  • Removed obsolete migration messaging from README.md to keep contributor guidance current and reduce noise.

- > [!IMPORTANT]
- > We've switch TO `main` and away from `master` branch
-
- > [!NOTE]
- > You may need to update your remote origin target in git.

• Fixes Update readme #920