issue templates: better version prompt

[smarter]

Almost no one uses scalac by hand, so it's not a good way to get the
current compiler version (example:
#11211), give the sbt command
instead.

[liufengyun]

We encourage programmers to minimize, using SBT for minimization is awkward. I think at least we should list scalac as a way to get compiler version.

[smarter]

It's much easier to minimize by using your normal build tool than by trying to replace it by scalac, and I think that if they're using scalac by hand, they know which version they've installed, but feel free to add a suggestion with the text you'd like to add here.

[smarter]

(https://github.com/scala/bug/blob/master/.github/ISSUE_TEMPLATE/bug.md doesn't list any command and it doesn't seem to be a problem)

[liufengyun]

If you want to add sbt and/or mill , maybe just add more lines to the code block.

[smarter]

Since the current situation is confusing (cf #11211), adding more lines of code is likely to make it more confusing I think, maybe there's a better way to describe things but I would rather remove things that add more things.

[liufengyun]

If a reporter only uses sbt, I think it's fair enough to add sbt "print scalaVersion" as a way to get compiler version. I think we can trust programmers who bother to report a bug can figure out the version if we give enough tips (I assume most of them are professional programmers).

Meanwhile, if they don't know scalac before, it's good to know and learn that as a Scala programmer --- a reward for bug report :)

[smarter]

OK, I've tried to find a wording which keeps both commands without being too confusing.

[liufengyun]

Thanks for taking care 👍

[liufengyun]

LGTM