Stop using forked magnolia

[nevillelyh]

https://github.com/spotify/scio/blob/master/build.sbt#L375

Just realized that we're still using a forked magnolia, and can't use latest upstream since they dropped 2.11 support. We should figure out a way forward and minimize custom stuff.

softwaremill/magnolia#171

One solution might be using our fork for 2.11, and latest upstream for 2.12, and drop 2.11 when we move to 2.13.

[regadas]

magnolia and it's dependency mercator are no longer published for 2.11 due to some weird scalac issues that pop up mainly (and this is the case i remember) when doing type class derivation for recursive types and when doing kind projection.

Given this, nothing really stop us from forking these two libs and publish them for 2.11 (I tested it briefly and it's doable).

Tbh, I tend to prefer this way as opposed to have two different versions. This is a bit more work, but at least things are bit more consistent.

[nevillelyh]

So latest magnolia HEAD no longer builds for scala 2.11 because:

• It depends on https://github.com/propensive/mercator 0.2.1 which is published for scala 2.12 & 2.13 only. Last release for scala 2.11 was 0.1.1
• There are breaking macro API changes

Do we want to keep patching both magnolia & mercator and release our forks? Feels like it's easier to use forked one for 2.11 and latest vanilla release for 2.12 & 2.13.

[error] /home/neville/src/oss/magnolia/core/shared/src/main/scala/magnolia.scala:214:64: value isJavaAnnotation is not a member of c.universe.Symbol
[error]       val isJavaAnnotation: Tree => Boolean = _.tpe.typeSymbol.isJavaAnnotation
[error]                                                                ^
[error] /home/neville/src/oss/magnolia/core/shared/src/main/scala/magnolia.scala:214:64: value isJavaAnnotation is not a member of c.universe.Symbol
[error]       val isJavaAnnotation: Tree => Boolean = _.tpe.typeSymbol.isJavaAnnotation