Upgrade assertj-core to version 3.15.0

[scordio]

No description provided.

[codecov-io]

Codecov Report

Merging #1867 into release/3.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##             release/3.x    #1867      +/-   ##
=================================================
- Coverage          86.87%   86.85%   -0.02%     
+ Complexity          2517     2516       -1     
=================================================
  Files                314      314              
  Lines               6619     6619              
  Branches             831      831              
=================================================
- Hits                5750     5749       -1     
  Misses               671      671              
- Partials             198      199       +1

Impacted Files	Coverage Δ	Complexity Δ
...ockito/internal/util/reflection/InstanceField.java	45.83% <0%> (-4.17%)	8% <0%> (-1%)

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[TimvdLippe]

Thank you!

[mockitoguy]

Nice upgrade, thanks!