Bump to sbt 1.12.9 (was 1.11.7)

https://github.com/sbt/sbt/releases/tag/v1.12.0 has a section titled
"changes with compatibility implications", but it doesn't seem to
contain anything relevant to us.

https://github.com/sbt/sbt/releases/tag/v1.12.7 is interesting because
it reportedly fixes
[CVE-2026-32948](GHSA-x4ff-q6h8-v7gw) on
Windows. I don't think this CVE is relevant to us at all (I believe it
would only apply to running sbt in an untrusted repo), but sbt rarely
gets any CVEs, so it's noteworthy.

Author: generalmimon

project/build.properties

@@ -1 +1 @@
-sbt.version = 1.11.7
+sbt.version = 1.12.9