Various small bug fixes

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• Fixes some small bugs that could result in PHP notices/warnings/errors.

Relevant technical choices:

Admin_Notifications::add_notification(): bug fix

The Admin_Notifications::add_notification() method only expects one parameter, so no need to ask WP for 2.

Downgrader::downgrade(): bug fix

The $adapter property does not exist. The $adapter local variable does.

Feature_Toggler::add_hooks(): use the correct function

The wpseo_enable_feature hook is a filter, not an action.

Plugin_Toggler::verify_nonce(): consistent return type

The return type is documented as bool, but the function contained a path which didn't return any value. Fixed now.

Admin_Page::add_assets(): bug fix

The third ($deps) parameter for wp_enqueue_style() expects an array of strings, not null.

Ref: https://developer.wordpress.org/reference/functions/wp_enqueue_style/

Plugin_Toggler::activate_plugin(): bug fix

The second ($redirect) parameter for activate_plugin() expects a string, not null.

Ref: https://developer.wordpress.org/reference/functions/activate_plugin/

Milestone

• I've attached the next release's milestone to this pull request.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

only this part can be tested: Downgrader::downgrade(): bug fix:

• Install and activate the latest Free and Test Helper
• edit the Free source file lib/migrations/adapter.php at line 1049 from

		if ( $this->in_transaction === false ) {

to

		if ( true || $this->in_transaction === false ) {

• visit Tools > Yoast Test
• in the "Downgrade Yoast SEO" section, downgrade to 20.7 and save
• with Test Helper versions up to 1.18-RC5, you would get a fatal error
• with Test Helper versions 1.18-RC6, the operation would work as expected.

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Fixes #

[enricobattocchi]

OK! Rebasing to the release branch, aiming to release it before the end of the year