KAFKA-9907: Switch default build to Scala 2.13 (#8537)

Scala 2.13.2 introduces support for suppressing warnings,
which makes it possible to enable fatal warnings. This is
useful enough from a development perspective to justify
this change.

In addition, Scala 2.13.2 also has a Vector implementation
with significant performance improvements and encoding
of String matches to switches.

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>

README.md

@@ -6,7 +6,7 @@ You need to have [Java](http://www.oracle.com/technetwork/java/javase/downloads/
 
 Java 8 should be used for building in order to support both Java 8 and Java 11 at runtime.
 
-Scala 2.12 is used by default, see below for how to use a different Scala version or all of the supported Scala versions.
+Scala 2.13 is used by default, see below for how to use a different Scala version or all of the supported Scala versions.
 
 ### Build a jar and run it ###
     ./gradlew jar

bin/kafka-run-class.sh

@@ -48,7 +48,7 @@ should_include_file() {
 base_dir=$(dirname $0)/..
 
 if [ -z "$SCALA_VERSION" ]; then
-  SCALA_VERSION=2.12.11
+  SCALA_VERSION=2.13.2
   if [[ -f "$base_dir/gradle.properties" ]]; then
     SCALA_VERSION=`grep "^scalaVersion=" "$base_dir/gradle.properties" | cut -d= -f 2`
   fi

bin/windows/kafka-run-class.bat

@@ -27,7 +27,7 @@ set BASE_DIR=%CD%
 popd
 
 IF ["%SCALA_VERSION%"] EQU [""] (
-  set SCALA_VERSION=2.12.11
+  set SCALA_VERSION=2.13.2
 )
 
 IF ["%SCALA_BINARY_VERSION%"] EQU [""] (

docs/js/templateData.js

@@ -20,5 +20,5 @@ var context={
     "version": "26",
     "dotVersion": "2.6",
     "fullDotVersion": "2.6.0",
-    "scalaVersion": "2.12"
+    "scalaVersion": "2.13"
 };

gradle.properties

@@ -21,6 +21,6 @@ group=org.apache.kafka
 #  - tests/kafkatest/version.py (variable DEV_VERSION)
 #  - kafka-merge-pr.py
 version=2.6.0-SNAPSHOT
-scalaVersion=2.12.11
+scalaVersion=2.13.2
 task=build
 org.gradle.jvmargs=-Xmx1024m -Xss2m

release.py

@@ -264,7 +264,7 @@ def command_stage_docs():
 
     cmd("Building docs", "./gradlew -Pversion=%s clean aggregatedJavadoc" % gradle_version_override, cwd=REPO_HOME, env=jdk8_env)
 
-    docs_tar = os.path.join(REPO_HOME, 'core', 'build', 'distributions', 'kafka_2.12-%s-site-docs.tgz' % gradle_version_override)
+    docs_tar = os.path.join(REPO_HOME, 'core', 'build', 'distributions', 'kafka_2.13-%s-site-docs.tgz' % gradle_version_override)
 
     versioned_docs_path = os.path.join(kafka_site_repo_path, docs_version(version))
     if not os.path.exists(versioned_docs_path):
@@ -663,8 +663,8 @@ Now you should sanity check it before proceeding. All subsequent steps start mak
 Some suggested steps:
 
  * Grab the source archive and make sure it compiles: https://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz
- * Grab one of the binary distros and run the quickstarts against them: https://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka_2.12-%(release_version)s.tgz
- * Extract and verify one of the site docs jars: https://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka_2.12-%(release_version)s-site-docs.tgz
+ * Grab one of the binary distros and run the quickstarts against them: https://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka_2.13-%(release_version)s.tgz
+ * Extract and verify one of the site docs jars: https://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka_2.13-%(release_version)s-site-docs.tgz
  * Build a sample against jars in the staging repo: (TODO: Can we get a temporary URL before "closing" the staged artifacts?)
  * Validate GPG signatures on at least one file:
       wget https://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz &&