feat(build): target JDK 1.8

BREAKING: downgrade from Java 11 to Java 8. Update your source code accordingly. This is because most servers still run Java 8 and most of the plugins out there do not support Java 11.

.github/workflows/gradle.yml

@@ -6,10 +6,10 @@ jobs:
     runs-on: ubuntu-18.04
 
     steps:
-      - name: Set up JDK 11
+      - name: Set up JDK 8
         uses: actions/setup-java@v1
         with:
-          java-version: 11
+          java-version: 8
 
       - name: Cache Gradle
         id: cache-gradle
@@ -48,10 +48,10 @@ jobs:
     runs-on: ubuntu-18.04
 
     steps:
-      - name: Set up JDK 11
+      - name: Set up JDK 8
         uses: actions/setup-java@v1
         with:
-          java-version: 11
+          java-version: 8
 
       - name: Set up Node.js v12.x
         uses: actions/setup-node@v1

build.gradle

@@ -15,8 +15,8 @@ plugins {
 apply from: "$rootDir/gradle/jacoco.gradle"
 apply from: "$rootDir/gradle/publish.gradle"
 
-sourceCompatibility = 11
-targetCompatibility = 11
+sourceCompatibility = 8
+targetCompatibility = 8
 
 ext {
     //Define one of the supported mc versions