[Feature Request] Migrate to Gradle

### Is your feature request related to a problem? Please describe.

Gradle is just better than Maven :D

### What is your ideal solution to the problem?

1. Remove Maven
2. Add Gradle

### How would this feature be used?

* If Gradle is not installed:
1. Run `./gradlew build`
2. The Gradle Wrapper automatically downloads and sets up the required Gradle version
3. The project builds successfully
* If Gradle is already installed:
1. Run `./gradlew build`
2. The Gradle Wrapper uses the project’s specified Gradle version (ignoring the system installation)
3. The project builds successfully

### Additional Info

_No response_

### Checklist

- [x] I have checked [the documentation](https://jmusicbot.com/) to make sure this feature doesn't already exist
- [x] I have searched for [similar feature requests](https://github.com/jagrosh/MusicBot/labels/enhancement)
- [x] I have confirmed that my requests is **not** on the [list of things that will not be added](https://jmusicbot.com/things-that-wont-be-added)
- [x] I am running the latest version of the bot: 

Start by locating the repository’s existing Maven build configuration and checking how the project is currently built. Use the requested `./gradlew build` flow as the validation entry point; done means Maven is removed, the Gradle Wrapper is included, and the project builds successfully.

Written by the indexing model from the issue text.