Drop Scala 2.12

[armanbilge]

#857 (comment)

[codecov-commenter]

Codecov Report

Merging #860 (1c1d77a) into main (96bf4e4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #860   +/-   ##
=======================================
  Coverage   83.88%   83.88%           
=======================================
  Files         126      125    -1     
  Lines        1756     1756           
  Branches      155      159    +4     
=======================================
  Hits         1473     1473           
  Misses        283      283           

Impacted Files	Coverage Δ
...hared/src/main/scala-2/codec/EnumCodecCompat.scala	100.00% <ø> (ø)
...s/core/shared/src/main/scala/codec/EnumCodec.scala	100.00% <ø> (ø)
modules/core/shared/src/main/scala/data/Arr.scala	100.00% <100.00%> (ø)