Bump Velocity to Java 11 minimum

Fixes #381. There is generally a consensus that we should move forward with this. We do not anticipate a Velocity 2.0.0 release until well after the release of the next Java LTS version (Java 17).

This commit doesn't clean up anything for the new Java version yet.

build.gradle

@@ -44,6 +44,11 @@ allprojects {
         }
     }
 
+    java {
+        sourceCompatibility = JavaVersion.VERSION_11
+        targetCompatibility = JavaVersion.VERSION_11
+    }
+
     repositories {
         mavenLocal()
         mavenCentral()