Update scala-library to 2.12.21 in series/0.5

http4s-steward[bot] · http4s-steward[bot] · commit 1a067d40b814 · 2025-12-12T01:05:01.000Z
diff --git a/build.sbt b/build.sbt
@@ -1,6 +1,6 @@
 import com.typesafe.tools.mima.core._
 
-val Scala212 = "2.12.20"
+val Scala212 = "2.12.21"
 val Scala213 = "2.13.18"
 
 inThisBuild(
