Bump plugin version to 0.9.9

[alecgeatches]

Bumps plugin version to 0.9.9 based off of these recent PR merges into develop:

• Test fix: Fix e2e tests, bump lowest supported PHP version to 7.4 (Fix failing GitHub e2e tests #722)
• Test fix: Update ESLint configuration and format JS files (Fix the failing linting step #723)
• Enhancements: Update WordPress minimum version to 5.9 & update PHPUnit Set WordPress minimum version to 5.9 & update PHPUnit #724 (Set WordPress minimum version to 5.9 & update PHPUnit #724)
• Enhancements: Move JS environment to node 20, upgrade packages (Update the JS packages #725)
• README updates from this branch: 579a365

[alecgeatches]

@ingeniumed Making a note here that I've had two test failures in a row:

• Attempt #1: Navigation timeout of 30000 ms exceeded during switchUserToAdmin() in
  tests/e2e/specs/simple.test.js.
• Attempt #2: Navigation timeout of 30000 ms exceeded during switchUserToAdmin() in
  tests/e2e/specs/editorial-comments.test.js.

In both cases, the failing function is switchUserToAdmin() from library @wordpress/e2e-test-utils. I'm unable to replicate locally. It's odd to suddenly see flaky tests after that run of 15 successful e2e tests in a row in #723.