Fix post-merge publishing (#2055)

* Check Unidoc on all versions of Scala

  It is required for publishing and we publish every version

* Fix conflicting cross-version suffixes issue

  When running `sbt ++2.13.4 unidoc`, SBT would set the Scala version
  for the fuzzer and benchmark projects even though they aren't really
  relevant to the command. This may be a misconfiguration or a bug in
  the unidoc plugin. Whatever the case, simply making it possible for
  them to use the same version of Scala as the firrtl project (on which
  they depend) fixes the issue.

* Match versions of Scala in build.sbt and CI

* Fix unidoc issues in 2.13.4

  There is some bug in ScalaDoc not finding some links in firrtl.options
  so those links were made absolute as a workaround.