[1.6.x] compiler: account for Scala 2 and 3 differences in generated varargs code (backport #737) by @ahjohannessen

[mergify]

This is an automatic backport of pull request #737 done by Mergify.
Cherry-pick of c9bef0a has failed:

On branch mergify/bp/1.6.x/pr-737
Your branch is up to date with 'origin/1.6.x'.

You are currently cherry-picking commit c9bef0a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   compiler/src/main/scala/play/twirl/compiler/TwirlCompiler.scala
	modified:   compiler/src/test/scala-2/play/twirl/compiler/test/Helper.scala
	modified:   compiler/src/test/scala-3/play/twirl/compiler/test/Helper.scala
	modified:   sbt-twirl/src/main/scala/play/twirl/sbt/SbtTwirl.scala
	modified:   sbt-twirl/src/main/scala/play/twirl/sbt/TemplateCompiler.scala

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .gitignore
	both modified:   compiler/src/main/java/play/japi/twirl/compiler/TwirlCompiler.java

Cherry-pick of ce5e0f1 has failed:

On branch mergify/bp/1.6.x/pr-737
Your branch is ahead of 'origin/1.6.x' by 3 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit ce5e0f1.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .gitignore

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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