Gradle: Enforce same JDK everywhere

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
         matrix:
             os: [ubuntu-latest, macOS-latest, windows-latest]
-            java: [ '11', '11.0.3', '15', '15.0.5']
+            java: ['11']
     name: Test Java ${{ matrix.Java }}, ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4.0.0

build-logic/commons/src/main/groovy/bisq.java-conventions.gradle

@@ -8,8 +8,13 @@ repositories {
     maven { url "https://jitpack.io" }
 }
 
-sourceCompatibility = JavaVersion.VERSION_11
-targetCompatibility = JavaVersion.VERSION_11
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(11)
+        vendor = JvmVendorSpec.AZUL
+        implementation = JvmImplementation.VENDOR_SPECIFIC
+    }
+}
 
 tasks.withType(JavaCompile) {
     options.encoding = 'UTF-8'