Revert to scala 2.12.12 due to scoverage issue with scala 2.12.13

Problem/Solution

The scoverage plugin has an issue due to a binary incompatibility introduced in scala 2.12.13
(see: scoverage/sbt-scoverage#319). It has been patched in the
plugin in scoverage/scalac-scoverage-plugin#306 which is currently
awaiting a release. However after updating our projects to scala 2.12.13, coverage is broken
and failing the CI builds. Downgrade to scala 2.12.12 until the scoverage plugin has been
published with the fix.

Differential Revision: https://phabricator.twitter.biz/D635917

.github/workflows/continuous-integration.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        scala: [2.12.13, 2.13.1]
+        scala: [2.12.12, 2.13.1]
         java: ['1.8', '1.11']
     runs-on: ubuntu-latest
     steps:

CHANGELOG.rst

@@ -10,6 +10,9 @@ Unreleased
 Runtime Behavior Changes
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+* util: Revert to scala version 2.12.12 due to https://github.com/scoverage/sbt-scoverage/issues/319
+  ``PHAB_ID=D635917``
+
 * util: Bump scala version to 2.12.13 ``PHAB_ID=D632567``
 
 Breaking API Changes

build.sbt

@@ -78,8 +78,8 @@ def jdk11GcJavaOptions: Seq[String] = {
 }
 
 val defaultProjectSettings = Seq(
-  scalaVersion := "2.12.13",
-  crossScalaVersions := Seq("2.12.13", "2.13.1")
+  scalaVersion := "2.12.12",
+  crossScalaVersions := Seq("2.12.12", "2.13.1")
 )
 
 val baseSettings = Seq(