Update sbt to 1.3.9

[scala-steward]

Updates org.scala-sbt:sbt from 1.3.8 to 1.3.9.

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

If you'd like to skip this version, you can just close this PR. If you have any feedback, just mention me in the comments below.

Have a fantastic day writing Scala!

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, semver-patch

[codecov-io]

Codecov Report

Merging #3376 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3376   +/-   ##
=======================================
  Coverage   91.22%   91.22%           
=======================================
  Files         379      379           
  Lines        7980     7980           
  Branches      218      218           
=======================================
  Hits         7280     7280           
  Misses        700      700           

Flag	Coverage Δ
#scala_version_212	91.20% <ø> (ø)
#scala_version_213	91.01% <ø> (ø)

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