Use sbt circe plugin

[lorandszakacs]

• title says it all, use the new sbt-circe-io plugin 😄. Depends on initial implementation of plugin sbt-circe-org#1
• removed configs keys that sbt was warning me about
• not entirely sure if Mima needs more config 🤷
• enables warnings, but disables fatal CI warning. We should fix all at some point
• use the sama scalafmt.conf as circe
• inherit scalafix w/ organizeImports from the sbt plugin
• ran the prePR command, which is the only thing that modified source code outside of the build

Also, the current build definitely uses the equivalent of ThisBuild / tlSonatypeUseLegacyHost := false (LE: I meant the exact opposite. Booleans are hard). Not sure if the circe org can publish to new hosts though.

I abstained from upgrading various versions in this PR 😅 I'll follow-up with that if thisi gets merged.

[lorandszakacs]

On hold until we get the org plugin out the door. After that I'll update this PR to use that instead (it's also how I'm testing the plugin locally)
circe/sbt-circe-org#1

[zmccoy]

Hi @lorandszakacs, do you have any more commits for this that you need to push up? Just want to make sure this gets over the line if someone needs to pick it up. Thanks!

[nafg]

Any way I can help with this?

[codecov-commenter]

Codecov Report

Patch coverage: 69.23% and project coverage change: +49.64 🎉

Comparison is base (799c1b9) 31.33% compared to head (dd2a40e) 80.98%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #423       +/-   ##
===========================================
+ Coverage   31.33%   80.98%   +49.64%     
===========================================
  Files           8        6        -2     
  Lines         584      163      -421     
  Branches       63       23       -40     
===========================================
- Hits          183      132       -51     
+ Misses        401       31      -370     

Flag	Coverage Δ
2.12.15	80.86% <69.23%> (?)
2.13.8	76.10% <61.53%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...o/circe/derivation/annotations/Configuration.scala	64.70% <ø> (ø)
...-2.12/io/circe/derivation/ScalaVersionCompat.scala	100.00% <ø> (ø)
...-2.13/io/circe/derivation/ScalaVersionCompat.scala	100.00% <ø> (ø)
.../src/main/scala/io/circe/derivation/renaming.scala	100.00% <ø> (ø)
...la/io/circe/derivation/annotations/JsonCodec.scala	70.58% <63.63%> (-5.61%)	⬇️
...n/scala/io/circe/derivation/DerivationMacros.scala	85.98% <100.00%> (+0.26%)	⬆️

... and 3 files with indirect coverage changes

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