Update scala-library to 2.12.20 (#508)

Co-authored-by: Alex Archambault <alexandre.archambault@gmail.com>

Author: scalameta-bot

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        scala: [2.12.19, 2.13.14]
+        scala: [2.12.20, 2.13.14]
         os: [ubuntu-latest]
         java: [11, 8]
     steps:

build.sbt

@@ -13,7 +13,8 @@ lazy val Version = new {
     "2.12.16",
     "2.12.17",
     "2.12.18",
-    "2.12.19"
+    "2.12.19",
+    "2.12.20"
   )
   def scala213 = scala213Versions.last
   def scala212 = scala212Versions.last