Update build.sbt when upgrading sbt version

[anilkumarmyla]

Currently, updating sbt version is only reflected in file project/build.properties but there might be usages of the same in build.sbt (sometimes as crossSbtVersions) which is not updated.

Any reason for this? Ex: cb372/sbt-explicit-dependencies#61 does not update the value in build.sbt

[mzuehlke]

As you mentioned sbt versions are currently only updated in files named build.properties see: https://github.com/fthomas/scala-steward/blob/master/modules/core/src/main/scala/org/scalasteward/core/io/package.scala#L31

It would be possible allow changes in "normal" source files to (as tested by isGenericSourceFile), but this could lead to too many changes. A problem that just happened with version numbers in text files (#1376).

Difficult decision.

[anilkumarmyla]

hmm, yeah. closing

[fthomas]

The file restriction is there since Scala Steward can update sbt versions (#665). I think we never tried updating sbt versions anywhere else.

If we allow Scala Steward to update sbt versions in other source files, it might produce a diff like cb372/sbt-explicit-dependencies@014427a (including the change in the .travis.yml) or it might produce more bad PRs (as @mzuehlke pointed out). Maybe we should give it a try.