Use latest WP in testing env

[jakejackson1]

Description

Instead of using a fixed value, always use the latest production release of WordPress.

Testing instructions

Automated tests will handle this.

Screenshots

Checklist:

• [X] I've tested the code.
• [ ] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [ ] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

Merging #1398 (0642d9f) into development (56df4b6) will not change coverage. The diff coverage is n/a.

@@              Coverage Diff               @@
##             development    #1398   +/-   ##
==============================================
  Coverage          77.32%   77.32%           
  Complexity          2896     2896           
==============================================
  Files                243      243           
  Lines              10096    10096           
  Branches             370      370           
==============================================
  Hits                7807     7807           
  Misses              2281     2281           
  Partials               8        8           

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