Update GH phpunit tests action and some node modules

[imath]

Update wp-phpunit to 6.1
Add WP 6.1 to the GH action tested versions
Update @wordpress/env to 5.6.0 and some other node modules.

Unit tests are broken when WP master is not used, let's see if updating env to 5.6.0 fixes the issue.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8759

[imath]

In 13355:

Update the GH action WP testing matrix, WP PHPunit and @wordpress/env

This commit also fixes:

• a GH action issue when testing WP < 6.1. In this case we need to downgrade WP PHPunit to a version lower than 6.1.0 to avoid @wordpress/env unit test command to fail (Internal Server Error).
• the database error when running the BP_Tests_Groups_Functions test.

Closes ​https://github.com/buddypress/buddypress/pull/34
Fixes #8759