[SCALA] Support scala 2.12 with Codegen 3.0

Description

When generating with -I scala, the resulting build.sbt has scalaVersion := "2.11.12". Manually updating this to 2.12.x results in a failure to find the "com.wordnik.swagger" %% "swagger-async-httpclient" % "0.3.5" dependency.

Swagger-codegen version

$ swagger-codegen version
17:40:00.117 [main] DEBUG io.swagger.codegen.v3.cli.SwaggerCodegen - there are not options for command 'langs'
17:40:00.118 [main] DEBUG io.swagger.codegen.v3.cli.SwaggerCodegen - there are not options for command 'version'
3.0.10

As there is a related issue for swagger 2.x, this is not a regression.

Swagger declaration file content or url

Any valid swagger file.

Command line used for generation

swagger-codegen generate -i gzrange-api.yaml -l scala

Steps to reproduce

Run swagger as above.
Edit build.sbt to have scalaVersion := "2.12.8" (or any 2.12.x)
sbt publishLocal

This will give an error for an unresolved dependency.

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/8520

Suggest a fix/enhancement

It appears that a replacement for swagger-async-httpclient is needed, or it needs to be rebuilt with scala 2.12.

Since scala 2.13 has also been released, a similar fix to generate scala 2.13 should also be applied.

Start by running the reported swagger-codegen generate command with -l scala and inspect the generated build.sbt, especially the Scala version and com.wordnik.swagger %% swagger-async-httpclient dependency. Check dependency availability for Scala 2.12 and 2.13, then verify that the generated project can publish locally with each supported version.