WIP: Enable explicit nulls in community build

[olhotak]

Enables -Yexplicit-nulls for those community build projects that work with it.

Depends on #13975.

Currently applies only to sbt projects, not mill projects, because I don't know how to set scalacOptions from outside the project's own build file for mill.

The following 10 projects work with just -Yexplicit-nulls:

• shapeless
• discipline
• disciplineSpecs2
• simulacrumScalafixAnnotations
• scissEqual
• scissFingerTree
• scissModel
• scissNumbers
• scissSpan
• akka

34 projects work with -Yexplicit-nulls and -language:unsafeNulls.

The following projects don't compile even with -language:unsafeNull:

• scalacheck
• scalatest
• scalatestplusScalacheck
• scalatestplusJunit
• scalatestplusTestNG
• scalaXml
• scalaPB
• stdLib213
• effpi
• sconfig
• scalaz
• catsEffect3
• onnxScala
• libretto
• jacksonModuleScala

A summary of the errors in some of the above projects is in this file:
dotty-cb-exp-null.txt

[ckipp01]

I'm going to go ahead and close to inactivity. @olhotak if this is still something you want to work on, feel free to comment back, rebase, and re-open!