Avoid unusual kind-projector syntax

[travisbrown]

Another Dotty-motivated change. These are the only two places we use kind-projector's *[_] syntax, which I personally find pretty unreadable, and I think in both cases writing out the context bounds is a big improvement.

[codecov-io]

Codecov Report

Merging #3186 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3186      +/-   ##
==========================================
- Coverage   92.99%   92.99%   -0.01%     
==========================================
  Files         376      376              
  Lines        7381     7380       -1     
  Branches      195      216      +21     
==========================================
- Hits         6864     6863       -1     
  Misses        517      517

Flag	Coverage Δ
#scala_version_212	93.32% <ø> (-0.01%)	⬇️
#scala_version_213	90.53% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Nested.scala	95.06% <ø> (-0.07%)	⬇️

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