Remove unsupported WP 6.1 build (#1491)

## Motivation for the change, related issues

Remove WP 6.1 from list of supported versions because our builds only
update current WP version minus 3.

Given `WP 6.5 - 3 = WP 6.2`, this means a breaking change like #1490
will not work on the stale WP 6.1 build on playground.wordpress.net.

So it is time to drop WP 6.1.

## Testing Instructions (or ideally a Blueprint)

- CI Tests

Author: brandonpayton