Added JAR manifest, moved version 21 -> 17

app/build.gradle

@@ -23,7 +23,7 @@ dependencies {
 // Apply a specific Java toolchain to ease working on different environments.
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(21)
+        languageVersion = JavaLanguageVersion.of(17)
     }
 }
 
@@ -31,3 +31,11 @@ application {
     // Define the main class for the application.
     mainClass = 'org.skinner.WebApp'
 }
+
+jar {
+  manifest {
+    attributes(
+      'Main-Class': 'org.skinner.WebApp'
+    )
+  }
+}