MINOR: Pass `-release 8` to scalac and upgrade to Gradle 6.4 (#8538)

The version of Zinc included with Gradle 6.4 includes a fix for the blocker
that was preventing us from passing `-release 8` to scalac.

Release notes for Gradle 6.4:

https://docs.gradle.org/6.4/release-notes.html

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

README.md

@@ -4,7 +4,9 @@ See our [web site](https://kafka.apache.org) for details on the project.
 
 You need to have [Java](http://www.oracle.com/technetwork/java/javase/downloads/index.html) installed.
 
-Java 8 should be used for building in order to support both Java 8 and Java 11 at runtime.
+We build and test Apache Kafka with Java 8, 11 and 14. We set the `release` parameter in javac and scalac
+to `8` to ensure the generated binaries are compatible with Java 8 or higher (independently of the Java version
+used for compilation).
 
 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.gradle

@@ -501,6 +501,10 @@ subprojects {
       ]
     }
 
+    // Scalac's `-release` requires Java 9 or higher
+    if (JavaVersion.current().isJava9Compatible())
+      scalaCompileOptions.additionalParameters += ["-release", minJavaVersion]
+
     configure(scalaCompileOptions.forkOptions) {
       memoryMaximumSize = '1g'
       jvmArgs = ['-Xss4m']

gradle/dependencies.gradle

@@ -61,7 +61,7 @@ versions += [
   bcpkix: "1.64",
   checkstyle: "8.20",
   commonsCli: "1.4",
-  gradle: "6.3",
+  gradle: "6.4",
   gradleVersionsPlugin: "0.28.0",
   grgit: "4.0.1",
   httpclient: "4.5.11",

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.4-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists