fix: support for Java 8 was dropped unexpectedly

GroovyCompile task still has no support for `--release` option, so
use the legacy way to configure the JavaVersion for source/target version.

build.gradle

@@ -9,6 +9,8 @@ plugins {
  id 'org.sonarqube' version '3.1.1'
 }
 
+sourceCompatibility = JavaVersion.VERSION_1_8
+targetCompatibility = JavaVersion.VERSION_1_8
 group = 'com.github.spotbugs.snom'
 
 repositories {
@@ -33,10 +35,6 @@ dependencies {
  testImplementation 'com.tngtech.archunit:archunit:0.16.0'
 }
 
-tasks.withType(JavaCompile).configureEach {
- options.release = 8
-}
-
 tasks.named('groovydoc') {
  docTitle 'SpotBugs Gradle Plugin'
  link 'https://docs.gradle.org/current/javadoc/', 'org.gradle.api.'