Update npm dependencies

[peterwilsoncc]

Description of the Change

Update npm dependencies, see change log.
Minor changes to the test suite to remove some flaky tests

How to test the Change

The test coverage on this is pretty good so I think it's good as is.

• Build, WP-Env, Cypress verified by passing E2E tests
• Run npm ci; npm run start
• Edit a JS file and ensure a build is triggered
• Edit a CSS file and ensure a build is triggered
• Run npm run lint:js and ensure eslint runs
• Run npm run plugin-zip and ensure a zip file is created

Changelog Entry

Changed - Update to Node version 20 for build and development tools.
Changed - Update npm dependencies via npm audit fix.
Developer - Update cypress to 15.9.0
Developer - Update @wordpress/scripts to 31.3.0
Developer - Update @wordpress/env to 10.38.0
Developer - Tests: Ensure images are saved to classic editor post content (flaky test fix).
Developer - Tests: Force click view post buttons in toolbar.

Credits

Props @peterwilsoncc

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.