Bump version of coverage-model to 0.20.0

[uhafner]

This PR bundles changes from coverage-model 0.20.0 and 0.21.0.

This update contains the following elements:

• Fixes https://github.com/jenkinsci/code-coverage-api-plugin/issues/598
• Introduces the maximum complexity metric, i.e. the maximum complexity of all methods.
• Introduces the metric tendency to allow quality gates based on all available metrics: some metrics are better when the value increases, and vice versa.

[codecov]

Codecov Report

Merging #615 (53c68f5) into master (d0f1efb) will decrease coverage by 0.24%.
The diff coverage is 27.27%.

@@             Coverage Diff              @@
##             master     #615      +/-   ##
============================================
- Coverage     72.24%   72.01%   -0.24%     
- Complexity     1593     1596       +3     
============================================
  Files           127      127              
  Lines          6126     6153      +27     
  Branches        645      654       +9     
============================================
+ Hits           4426     4431       +5     
- Misses         1483     1505      +22     
  Partials        217      217              

Impacted Files	Coverage Δ
...ns/coverage/metrics/steps/CoverageBuildAction.java	72.80% <0.00%> (-8.46%)	⬇️
...ins/coverage/metrics/steps/CoverageTableModel.java	7.97% <0.00%> (-0.42%)	⬇️
...overage/metrics/steps/CoverageChecksPublisher.java	83.40% <50.00%> (ø)
...ugins/coverage/metrics/model/ElementFormatter.java	51.09% <66.66%> (+1.09%)	⬆️
...ge/metrics/steps/CoverageQualityGateEvaluator.java	100.00% <100.00%> (ø)
...ugins/coverage/metrics/steps/CoverageRecorder.java	79.54% <100.00%> (+0.11%)	⬆️

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