Fix SQL performance issue on the student reports page

[m1r0]

Fixes #6122

Changes proposed in this Pull Request

The fix hints to the SQL optimizer to use the sensei_comment_type_user_id index. This is the default behavior on newer versions of MySQL/MariaDB but older versions use the woo_idx_comment_type index which leads to major performance issues.

The problem was spotted on a website using MariaDB 10.4.25.

Testing instructions

• Make sure the Reports -> Students screen works as before and there are no performance issues.

The lovely folks at Team 51 confirmed that this fixes the issue on their client's site. For more info, check this thread below.

Context

p1668540147192489-slack-C02P7FHLVR9

[codecov]

Codecov Report

Merging #6134 (dd354a6) into trunk (fa8bbe2) will increase coverage by 0.82%.
The diff coverage is 78.04%.

@@             Coverage Diff              @@
##              trunk    #6134      +/-   ##
============================================
+ Coverage     44.97%   45.80%   +0.82%     
- Complexity     9455     9472      +17     
============================================
  Files           476      476              
  Lines         33483    33543      +60     
  Branches        272      272              
============================================
+ Hits          15059    15363     +304     
+ Misses        18222    17978     -244     
  Partials        202      202              

Impacted Files	Coverage Δ
includes/class-sensei-posttypes.php	11.13% <ø> (+0.04%)	⬆️
includes/class-sensei-question.php	12.02% <0.00%> (-0.08%)	⬇️
...sensei-reports-overview-data-provider-students.php	80.76% <ø> (ø)
...t-api/class-sensei-rest-api-lessons-controller.php	7.69% <ø> (-2.61%)	⬇️
includes/class-sensei-lesson.php	37.11% <50.00%> (ø)
includes/class-sensei-modules.php	38.23% <86.20%> (+2.63%)	⬆️
includes/class-sensei-quiz.php	60.45% <100.00%> (+0.07%)	⬆️
includes/class-sensei-usage-tracking.php	25.00% <100.00%> (-0.75%)	⬇️
...api/class-sensei-rest-api-questions-controller.php	84.15% <100.00%> (ø)
.../class-sensei-learners-admin-bulk-actions-view.php	54.28% <0.00%> (-0.26%)	⬇️
... and 31 more

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