Prepare for renaming master -> main (#13235)

This commit replaces all references to PowerToys' master branch
with "main" in documentation, code and build rules.

- [x] **Linked issue:** #5433
- [x] **Communication:** I've discussed this with core contributors in
  the issue. 
- [ ] **Tests:** Added/updated and all pass (not applicable)
- [ ] **Installer:** Added/updated and all pass (not applicable)
- [ ] **Localization:** All end user facing strings can be localized
  (not applicable)
- [x] **Docs:** Updated
- [x] **Binaries:** Any new files are added to WXS / YML
   - [x] No new binaries