Upgrade to sbt 1.9.1.

sjrd · sjrd · commit 33e25a4501cd · 2023-07-06T18:00:03.000+02:00
So that we get its dual-publishing of sbt plugins.
diff --git a/project/build.properties b/project/build.properties
@@ -1 +1 @@
-sbt.version=1.5.4
+sbt.version=1.9.1
