Replace kind-projector placeholders in infix types

[travisbrown]

This is a subset of #3209 that only changes the use of kind-projector's * placeholder in infix types. I added support for * in function and tuple types to my -Ykind-projector proposal for Dotty, but infix types seem like more of a hassle and I don't think it's worth bothering with.

[codecov-io]

Codecov Report

Merging #3215 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3215   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         376      376           
  Lines        7412     7412           
  Branches      200      192    -8     
=======================================
  Hits         6897     6897           
  Misses        515      515

Flag	Coverage Δ
#scala_version_212	93.35% <100%> (-0.03%)	⬇️
#scala_version_213	92.82% <100%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Ior.scala	98.52% <100%> (ø)	⬆️
core/src/main/scala/cats/evidence/Is.scala	91.66% <100%> (ø)	⬆️

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