bump Scalameta, Scalafix, sbt versions

[SethTisue]

replaces a bunch of steward PRs — easier to land it all together as the Scalameta & Scalafix updates aren't independent

(and then the sbt one is just a bonus)

• Scalameta 4.6.0 (was 4.5.4)
• Scalafix 0.10.4 (was 0.9.24)
• sbt 1.7.3 (was 1.7.1)