Update sbt to 1.6.1

[scala-steward]

Updates org.scala-sbt:sbt from 1.5.8 to 1.6.1. GitHub Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

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Ignore future updates

Add this to your `.scala-steward.conf` file to ignore future updates of this dependency: ``` updates.ignore = [ { groupId = "org.scala-sbt", artifactId = "sbt" } ] ```

labels: library-update, early-semver-minor, semver-spec-minor, commit-count:1

[SethTisue]

@eed3si9n it looks like this might be some incompatibility between sbt 1.6.1 and Scala 2.12.4 specifically

if I only care about this repo, I would avoid the fishing expedition and just drop 2.12.4 (and 0 through 3 too) from the crossbuild... I really don't think genjavadoc needs to keep supporting weird old 2.12.x versions

but I thought I'd check first if this failure interests you

[error] ## Exception when compiling 9 sources to /home/runner/work/genjavadoc/genjavadoc/target/scala-2.12/classes
[error] java.lang.StackOverflowError
[error] scala.reflect.macros.Attachments.get(Attachments.scala:42)
[error] xsbt.Compat$OriginalTreeTraverser$Reflective$.traverseOriginal(Compat.scala:48)
[error] xsbt.Compat.processOriginalTreeAttachment(Compat.scala:25)
[error] xsbt.ExtractUsedNames$ExtractUsedNamesTraverser.handleClassicTreeNode(ExtractUsedNames.scala:294)
[error] xsbt.ExtractUsedNames$ExtractUsedNamesTraverser.traverse(ExtractUsedNames.scala:170)
[error] xsbt.ExtractUsedNames$ExtractUsedNamesTraverser.$anonfun$handleClassicTreeNode$3(ExtractUsedNames.scala:294)

[scala-steward]

Superseded by #310.