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']
         project: ['finagle-base-http', 'finagle-benchmark', 'finagle-benchmark-thrift', 'finagle-core', 'finagle-example', 'finagle-exception', 'finagle-exp', 'finagle-grpc-context', 'finagle-http', 'finagle-http2','finagle-init','finagle-integration','finagle-memcached','finagle-mux','finagle-mysql','finagle-netty4','finagle-netty4-http','finagle-opencensus-tracing','finagle-partitioning','finagle-redis','finagle-scribe','finagle-serversets','finagle-stats','finagle-stats-core','finagle-thrift','finagle-thriftmux','finagle-toggle','finagle-tunable','finagle-zipkin-core','finagle-zipkin-scribe']
     runs-on: ubuntu-latest

CHANGELOG.rst

@@ -34,6 +34,9 @@ Breaking API Changes
 Runtime Behavior Changes
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+* finagle: Revert to scala version 2.12.12 due to https://github.com/scoverage/sbt-scoverage/issues/319
+  ``PHAB_ID=D635917``
+
 * finagle: Bump scala version to 2.12.13 ``PHAB_ID=D632567``
 
 * finagle-core: Move helper tracing methods like `traceLocal` in `Trace` into the `Tracing` class. This

build.sbt

@@ -138,8 +138,8 @@ def jdk11GcJavaOptions: Seq[String] = {
 val sharedSettings = Seq(
   version := releaseVersion,
   organization := "com.twitter",
-  scalaVersion := "2.12.13",
-  crossScalaVersions := Seq("2.12.13", "2.13.1"),
+  scalaVersion := "2.12.12",
+  crossScalaVersions := Seq("2.12.12", "2.13.1"),
   fork in Test := true, // We have to fork to get the JavaOptions
   libraryDependencies ++= Seq(
     // See https://www.scala-sbt.org/0.13/docs/Testing.html#JUnit

doc/src/sphinx/code/client-server-anatomy/build.sbt

@@ -2,7 +2,7 @@ name := "client-server-anatomy"
 
 version := "1.0"
 
-scalaVersion := "2.12.13"
+scalaVersion := "2.12.12"
 
 libraryDependencies ++= Seq(
   "com.twitter" %% "finagle-core" % "21.2.0"

doc/src/sphinx/code/quickstart/build.sbt

@@ -2,7 +2,7 @@ lazy val root = (project in file(".")).
   settings(
     inThisBuild(List(
       organization := "com.twitter",
-      scalaVersion := "2.12.13",
+      scalaVersion := "2.12.12",
       version      := "1.0"
     )),
     name := "quickstart",