build(deps): bump io.javalin:javalin-bundle in /backend

Bumps [io.javalin:javalin-bundle](https://github.com/javalin/javalin) from 5.6.1 to 6.1.6.
- [Release notes](https://github.com/javalin/javalin/releases)
- [Commits](javalin/javalin@javalin-parent-5.6.1...javalin-parent-6.1.6)

---
updated-dependencies:
- dependency-name: io.javalin:javalin-bundle
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/build.gradle

@@ -26,7 +26,7 @@ dependencies {
 
     // Javalin is a server/networking library for Java.
     implementation 'io.javalin:javalin:5.6.1'
-    implementation 'io.javalin:javalin-bundle:5.6.1'
+    implementation 'io.javalin:javalin-bundle:6.1.6'
     implementation 'org.slf4j:slf4j-simple:1.7.30'
     implementation 'org.json:json:20231013'
     implementation 'com.j2html:j2html:1.4.0'