2.12 build is broken

[SethTisue]

we aren't testing the 2.12 build in CI (probably inadvertently?), and so it bitrotted:

[error] /Users/tisue/Work/jardiff/core/src/test/scala/scala/tools/jardiff/IOUtilSpec.scala:9:8: object Using is not a member of package util
[error] import scala.util.Using
[error]        ^
[error] /Users/tisue/Work/jardiff/core/src/test/scala/scala/tools/jardiff/IOUtilSpec.scala:16:5: not found: value Using
[error]     Using.resource(new ZipOutputStream(Files.newOutputStream(jar)))(_.closeEntry()) // create jar
[error]     ^
[error] /Users/tisue/Work/jardiff/core/src/test/scala/scala/tools/jardiff/IOUtilSpec.scala:9:19: Unused import
[error] import scala.util.Using
[error]                   ^
[error] three errors found

this looks fixable by adding a dependency on scala-collection-compat, which iirc includes a 2.12 backport of Using

and then our CI config needs to be updated to actually ensure that the 2.12 build stays compilable

I wondered initially why we even bothered to crossbuild to both 2.12 and 2.13; why not drop 2.12, or even switch to Scala 3?

my best guess is that it probably has to do with #4 and #52 , an effort to make an sbt plugin; since sbt 1 is on Scala 2.12, that's what the 2.12 build would be for?

anyway, maybe a volunteer would like to tackle this