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/testing@3.1.7/packages/testing)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -49,7 +49,7 @@
     "@commitlint/cli": "^12.1.4",
     "@commitlint/config-conventional": "^12.1.4",
     "@open-wc/eslint-config": "^4.3.0",
-    "@open-wc/testing": "^2.5.33",
+    "@open-wc/testing": "^3.1.7",
     "@web/dev-server": "^0.1.22",
     "@web/test-runner": "^0.13.17",
     "@web/test-runner-playwright": "^0.8.8",