Gh-3157: Fix PMD plugin unnecessary warning

[GCHQDeveloper314]

Also enables the cache and bumps plugin version to latest

Related issue

• Resolve #3157

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.86%. Comparing base (b83b169) to head (7e7f154).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #3217   +/-   ##
==========================================
  Coverage      66.86%   66.86%           
  Complexity      2557     2557           
==========================================
  Files            912      912           
  Lines          29276    29276           
  Branches        3265     3265           
==========================================
  Hits           19576    19576           
  Misses          8256     8256           
  Partials        1444     1444           

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[GCHQDeveloper314]

I had upgraded the PMD plugin version to 3.22.0, but this results in version 7.0.0 of PMD being used which causes the build to fail because of new code checking problems which didn't occur with the previous PMD version. Using plugin version 3.21.0 instead pulls in PMD version 6.55.0 which doesn't introduce new failures.

A new issue should be raised to move to the latest PMD and address any code quality issues it raises.