Bump minimum required PHP version to 7.4

[m1r0]

Proposed Changes

• Bump minimum required PHP version to 7.4
• Bump minimum supported WP version for phpcs.

Testing Instructions

1. The CI should be passing.
2. Make sure all PHP 7.3 codebase mentions are replaced.

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 #7195 (349453d) into trunk (f8a80d0) will not change coverage.
Report is 29 commits behind head on trunk.
The diff coverage is 91.14%.

@@            Coverage Diff            @@
##              trunk    #7195   +/-   ##
=========================================
  Coverage     49.83%   49.83%           
  Complexity    10684    10684           
=========================================
  Files           601      601           
  Lines         45160    45160           
  Branches        402      402           
=========================================
  Hits          22504    22504           
  Misses        22329    22329           
  Partials        327      327           

Files	Coverage Δ
includes/class-sensei-dependency-checker.php	0.00% <ø> (ø)
includes/class-sensei-utils.php	52.12% <100.00%> (ø)
...ails/generators/class-student-completes-lesson.php	100.00% <100.00%> (ø)
...on/migrations/class-student-progress-migration.php	82.97% <100.00%> (ø)
...t-reading-aggregate-course-progress-repository.php	100.00% <100.00%> (ø)
...e-reading-aggregate-course-progress-repository.php	94.87% <100.00%> (ø)
.../class-tables-based-course-progress-repository.php	100.00% <100.00%> (ø)
...t-reading-aggregate-lesson-progress-repository.php	100.00% <100.00%> (ø)
...e-reading-aggregate-lesson-progress-repository.php	96.00% <100.00%> (ø)
.../class-tables-based-lesson-progress-repository.php	99.35% <100.00%> (ø)
... and 19 more

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[Imran92]

Maybe we need to update these requirements first to be able to merge this one

[Imran92]

LGTM 🪨