chore: don't run danger on main (#4079)

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`

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> CI-only branch filter on the Danger job; no application or release
pipeline behavior changes.
> 
> **Overview**
> The **`danger`** workflow’s `revenuecat/danger` job now has
**`branches: ignore: main`**, so CircleCI no longer runs Danger (and the
post-step **`api_diff_report_test.rb`**) on pushes to **`main`**.
> 
> Danger is meant to comment on pull requests; on **`main`** there is no
PR, so those runs were wasted. Feature and release branches are
unaffected unless they are **`main`**.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
35843e3aed9866a6e56d06e0fb766eabaa6b3de6. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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