Update scala-compiler, scala-library, ... to 2.12.16 (backport #2618) (#2624)

* Update scala-compiler, scala-library, ... to 2.12.16 (#2618)

* Update scala-compiler, scala-library, ... to 2.12.16
* Add 2.12.16 to compiler plugin crossbuild

Co-authored-by: Jack Koenig <jack.koenig3@gmail.com>
(cherry picked from commit e4f66357

)

# Conflicts:
#	.github/workflows/test.yml

* Resolve backport conflicts

* Ignore lack of plugin 3.5.3 artifact for Scala 2.12.16

Since 2.12.16 did not exist when 3.5.3 was published, there is no
artifact for the plugin so we must ignore it until 3.5.4 is released.

Co-authored-by: Scala Steward <43047562+scala-steward@users.noreply.github.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>