Housekeeping: remove unneeded legacy version checks & clean up code.

[Moderocky]

Description

This is the batch merge request for #6641.

It removes checks for methods existing that are standard as of 1.13.2, since we no longer support versions below this.

What can be removed?

Version checks for 1.13 and lower. Method & class existence checks that were standard as of Spigot for 1.13.2. Paper-only checks are still required for a few things that didn't arrive in Spigot until later (or at all).

See the linked issue #6641 for more details.

If you want to contribute to this project, open a pull request targeting remove-legacy-code and it will be included in this pull request.

What next?

Another clean-up can be done before 2.10 for things under 1.17. This will include a few bits and pieces that couldn't be fully cleaned up in this PR.

This is a branch merge request!

Individual pull requests were made by contributors to its feature branch: this can be merged as-is to preserve the Git change history.

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Target Minecraft Versions: any Requirements: none Related Issues: #6641