build(deps): bump flat and @open-wc/testing

Removes [flat](https://github.com/hughsk/flat). It's no longer used after updating ancestor dependency [@open-wc/testing](https://github.com/open-wc/open-wc/tree/HEAD/packages/testing). These dependencies need to be updated together.


Removes `flat`

Updates `@open-wc/testing` from 2.5.33 to 3.1.7
- [Release notes](https://github.com/open-wc/open-wc/releases)
- [Changelog](https://github.com/open-wc/open-wc/blob/master/packages/testing/CHANGELOG.md)
- [Commits](https://github.com/open-wc/open-wc/commits/@open-wc/[email protected]/packages/testing)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: "@open-wc/testing"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>