Fix PHP 8.1 deprecation notice on Students page

[donnapep]

Resolves #7029.

Proposed Changes

wp_kses was the culprit because null was being passed as the first parameter. This is because we were passing the result of the call to render_bulk_action_select_box, which has no return value, so null was used instead. The net result was that wp_kses had no effect.

This PR changes the implementation of render_bulk_action_select_box (and renames it) such that the HTML is returned. Now, null is no longer passed to wp_kses and the call actually works. This became apparent when I realized that the dropdown wasn't displaying correctly, because the class attribute was being stripped out. So I added class as part of the allowed HTML for wp_kses.

Testing Instructions

1. On a PHP 8.1 site, browse to the Students page.
2. Ensure the deprecated notice isn't being logged / displayed.
3. Check that the bulk action dropdown looks good and is populated correctly.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7038 (b664404) into trunk (5cbcc2e) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##              trunk    #7038      +/-   ##
============================================
- Coverage     48.03%   48.02%   -0.01%     
  Complexity    10458    10458              
============================================
  Files           573      573              
  Lines         44141    44145       +4     
  Branches        402      402              
============================================
  Hits          21201    21201              
- Misses        22613    22617       +4     
  Partials        327      327              

Impacted Files	Coverage Δ
.../class-sensei-learners-admin-bulk-actions-view.php	55.47% <0.00%> (-0.86%)	⬇️

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[Imran92]

Thanks for the explanation 💯 And it's working nicely 🚀