Test against WordPress 5.9

[jeffpaul]

Is your enhancement related to a problem? Please describe. Once WordPress 5.9 is released, we'll want to test WP-CLI Vulnerability Scanner to see if any incompatibility issues arise.

Describe the solution you'd like

• [x] test WP-CLI Vulnerability Scanner on WordPress 5.9
• [ ] open issues for any incompatibilities noted in testing
• [ ] resolve issues identified in testing
• [ ] bump "tested up to" version
• [ ] if code changes needed due to incompatibilities, ship a plugin release, otherwise use "Plugin asset/readme update" action to update "tested up to" version on .org repo

Designs n/a

Describe alternatives you've considered none

Additional context Related: n/a.

[jeffpaul]

From @kojraai:

Every functionality that I’ve been tested in the past (vulnerable plugin, rate limit … etc) is working as expected. Tested on 5.9.2. Looks good to me!