Set Java compatibility level to 16 (#3)

* Set Java compatibility level to 16

* Bump version to 3.1.1

build.gradle.kts

@@ -6,7 +6,7 @@ plugins {
 }
 
 group = "org.quiltmc"
-version = "3.1.0"
+version = "3.1.1"
 
 repositories {
     mavenCentral()
@@ -37,6 +37,11 @@ gradlePlugin {
     }
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_16
+    targetCompatibility = JavaVersion.VERSION_16
+}
+
 publishing {
     repositories {
         mavenLocal()
@@ -51,4 +56,4 @@ publishing {
             }
         }
     }
-}
+}