Upgrade to Javalin 7 and refactor route registration

Updated the dependency to Javalin v7.2.3 and refactored route registration to use the new `RoutesConfig` API. Adjusted `GuildRoutes`, `ActionRoutes`, and `WebApi` to align with updated conventions and centralized route definitions. Improved modularity, reduced redundancy, and ensured backward compatibility by migrating exception and error handler logic. Updated tests to comply with the refactored APIs and structure.

pom.xml

@@ -162,7 +162,7 @@
         <dependency>
             <groupId>io.javalin</groupId>
             <artifactId>javalin</artifactId>
-            <version>6.7.0</version>
+            <version>7.2.3</version>
         </dependency>
 
         <dependency>
@@ -202,7 +202,7 @@
             <artifactId>gitlab4j-api</artifactId>
             <version>6.2.0</version>
             <exclusions>
-                <!-- Clashes with the Servlet 5 API used by Javalin's Jetty 11 in the shaded jar. -->
+                <!-- Clashes with the Servlet API used by Javalin's Jetty 12 in the shaded jar. -->
                 <exclusion>
                     <groupId>jakarta.servlet</groupId>
                     <artifactId>jakarta.servlet-api</artifactId>