Removing the manuell download links and replacing them with an automatic system

[DerEingerostete]

This pull request includes:

• [ ] breaking changes
• [X] no breaking changes

Changes made to the repository:

I removed the manuell download links for GetBukkit.org and replaced them with a system that automaticaly gets all versions that can be downloaded from GetBukkit.org.

Documentation of test results:

Just tested if the funktion works as intended and if performance is okay (it worked and I improved performance after testing)

Related issues/discussions:

No related issues

[ghost]

DeepCode's analysis on #4e8f8e found:

• :x: 1 critical issue, :information_source: 1 minor issue. :point_down:

Top issues

Description	Example fixes
Unsanitized input from data from a remote resource flows into openConnection, where it is used as an URL to perform a request. This may result in a Server-Side Request Forgery vulnerability. Occurrences: SetupSpigotVersion.java:43	:wrench: Example fixes
Printing the stack trace of java.io.IOException. Production code should not use printStackTrace. Occurrences: GetBukkitVersion.java:42 GetBukkitVersions.java:78	:wrench: Example fixes

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