add WP and PHP minimum badges

[jeffpaul]

Description of the Change

This PR adds badges to note the WordPress and PHP minimums for this plugin. I did NOT remove the "Requirements" section of the README.md file, but I wonder if that might be worthwhile so that we don't need to worry about remembering to update those references in the future (as the badges auto-update based on what's shown on the WP.org repo)?

How to test the Change

Use GitHub markdown previewer to see updated badges.

Changelog Entry

n/a

Credits

Props @jeffpaul.

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.

[dkotter]

I did NOT remove the "Requirements" section of the README.md file, but I wonder if that might be worthwhile so that we don't need to worry about remembering to update those references in the future

I'm not strongly opinionated either way on this one. I personally feel the "Requirements" section is easier to see and parse as opposed to this long list of badges, so I like having both but it does duplicate that information so does also make sense to just remove it.