Update scala-library to 2.12.21

Author: scala-steward

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        scala: [2.11.12, 2.12.20, 2.13.18, 3.7.4]
+        scala: [2.11.12, 2.12.21, 2.13.18, 3.7.4]
         java: [temurin@11, temurin@17]
     runs-on: ${{ matrix.os }}
     steps:

project/ScalaVersions.scala

@@ -1,6 +1,6 @@
 object ScalaVersions {
   val scala211 = "2.11.12"
-  val scala212 = "2.12.20"
+  val scala212 = "2.12.21"
   val scala213 = "2.13.18"
   val scala3   = "3.7.4"