Update javalinVersion to v6

Author: renovate[bot]

javalin-demo/gradle.properties

@@ -4,7 +4,7 @@ org.gradle.parallel=true
 org.gradle.caching=true
 
 # Code
-javalinVersion=5.6.3
+javalinVersion=6.1.3
 webjarredocVersion=2.1.3
 unrestJavaVersion=3.14.5
 prometheusVersion=0.16.0