Deprecate and make private syntax methods that are now on Either

[travisbrown]

All of these methods are available directly on Either now that we've dropped 2.11. We could probably just deprecate them to discourage explicit usage but I package-privated them as well since it might have some tiny effect on compile times.

[codecov-io]

Codecov Report

Merging #3149 into master will decrease coverage by 0.06%.
The diff coverage is 10%.

@@            Coverage Diff             @@
##           master    #3149      +/-   ##
==========================================
- Coverage   93.28%   93.21%   -0.07%     
==========================================
  Files         376      376              
  Lines        7323     7323              
  Branches      195      195              
==========================================
- Hits         6831     6826       -5     
- Misses        492      497       +5

Flag	Coverage Δ
#scala_version_212	93.55% <10%> (-0.08%)	⬇️
#scala_version_213	90.86% <10%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/either.scala	83.08% <10%> (-3.68%)	⬇️

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