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 ElasticPress Stream to see if any incompatibility issues arise.

Describe the solution you'd like

• [ ] test ElasticPress Stream 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

Designs n/a

Describe alternatives you've considered none

Additional context n/a

[BBerg10up]

Attempted to test this today, but couldn't get it working. Following the readme, it seemed like it should've been easy to setup but I couldn't get the plugin working. Logged an issue: https://github.com/10up/elasticpress-stream/issues/6

[jeffpaul]

Note that I'm updating this from testing against WP 5.8 to 5.9.