update WP tested-up-to version 5.7 in readme.txt

[jeffpaul]

Co-Authored-By: Ankit K Gupta ankit.himcs@gmail.com

Description of the Change

Following on from #68 and applying the readme.txt change only to master to trigger the Plugin asset/readme update action and have this flow to WP.org SVN.

Alternate Designs

Wait until a formal release to have this change flow to WP.org SVN.

Benefits

Ensures we show the correct "tested up to" value on WP.org.

Possible Drawbacks

none identified.

Verification Process

n/a

Checklist:

• [x] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests passed.

Applicable Issues

Relates to #65.

Changelog Entry

n/a