Various updates for branch rename from master to 3.x

[jrfnl]

Description

• Ensure Dependabot still pulls GH Actions related changes to the 3.x branch, as CI for 3.x needs to keep working for another year.
• Ensure the "happy new year" PR goes to the 3.x branch, as CI for 3.x needs to keep working for another year.
• Update branch references for when to run workflows on push.
• Remove Composer branch alias which should no longer be necessary.
• Update README badges branch references.
• Update branch references and related instruction in issue/PR templates/CONTRIBUTING/release checklist.

Suggested changelog entry

• The master branch has been renamed to 3.x and the default branch has changed to the 4.x branch.
  • If you contribute to PHP_CodeSniffer, you will need to update your local git clone.
  • If you develop against PHP_CodeSniffer and run your tests against dev branches of PHPCS, you will need to update your workflows.

Related issues/external references

Related to #3

[jrfnl]

This is getting very annoying. This PR should have been merged straight after the branch rename with a tag to follow soon after, but I'm not tagging with a red cross on the builds. Tempted to temporarily disable Coveralls checks completely until they get their act together, but will give it a few more hours.

[jrfnl]

Finally.... build passing.