Address deprecations in FeedbackTest

[saeltz]

These deprecations came into effect with Scala 2.12. Since we dropped support for Scala 2.11, I did the changes.

Also removing some specific scalac options for not any longer supported patch versions of Scala 2.13.

[codecov-commenter]

Codecov Report

Base: 87.82% // Head: 87.11% // Decreases project coverage by -0.70% ⚠️

Coverage data is based on head (edb0c10) compared to base (98cfc7e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   87.82%   87.11%   -0.71%     
==========================================
  Files         141      141              
  Lines        1519     1521       +2     
  Branches      241      221      -20     
==========================================
- Hits         1334     1325       -9     
- Misses        185      196      +11     

Impacted Files	Coverage Δ
...pections/collections/PredefIterableIsMutable.scala	14.28% <100.00%> (-85.72%)	⬇️
...tions/collections/PredefTraversableIsMutable.scala	14.28% <100.00%> (-69.05%)	⬇️

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