Bump to StructArmed ^0.15 and enable MustBeFinalRule on tests

[samsonasik]

What was changed

StructArmed 0.15.0 introduce ExtendedClassAwareRuleInterface that makes MustBeFinalRule safely be used to skip if there is class that extend target class.

This PR bump StructArmed to ^0.15 and apply the MustBeFinalRule with single command to fix.

vendor/bin/structarmed analyze --fix

Why?

Make use of final class when possible.

Checklist

• Tested
  • Tested manually

Documentation

[samsonasik]

@roxblnfk ready to review/merge 👍

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.64%. Comparing base (e291586) to head (7f8fe8e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1294      +/-   ##
============================================
- Coverage     90.65%   90.64%   -0.02%     
  Complexity     6703     6703              
============================================
  Files           876      876              
  Lines         18978    18978              
============================================
- Hits          17204    17202       -2     
- Misses         1774     1776       +2     

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