GH actions need to update to `main` branch

[nwoodruff-co]

Lots point to master, but we renamed the branch to main. @PolicyEngine can you fix this with a pr please editing the GH actions (and make sure they run on changes to their own workflow file too)

[policyengine]

✅ Done! I've updated all GitHub Actions workflows to use the main branch instead of master.

Changes made:

• Updated 5 workflow files to reference main instead of master:
  • .github/workflows/code_changes.yaml (including comment)
  • .github/workflows/deploy.yml
  • .github/workflows/pr_code_changes.yaml
  • .github/workflows/pr_docs_changes.yaml
  • .github/workflows/versioning.yaml

Workflow file triggers:
All workflows already include .github/** in their path filters (or run on all pushes for deploy.yml), so they will automatically run on changes to workflow files.

Note:
The lgeiger/black-action@master references remain unchanged - these refer to the action's repository branch, not this repository's branch.

Created PR #1471 for review.