chore: don't run danger on main (#1842)

Danger runs on every push to `main`, but there's no PR there for it to
comment on.

Added a branch filter to the danger job so it skips `main`.
`cordova-plugin-purchases` already did this, this brings the rest of the
SDK repos in line. Release and snapshot branches are unchanged.

### Verification
- [x] `circleci config validate .circleci/config.yml`

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>