Set mappings namespace in manifest

[Warriorrrr]

Description:

Since we don't do any intensive reflection into internals, the plugin doesn't need any remapping done, setting the manifest entry prevents that.

Also some other general pom file maintenance:

• Removed exclusion for the bukkit api from vault api, 1.7.1 no longer exposes it
• Removed commons text, we don't use it anywhere
• Changed inclusions to exclusions in the shade plugin
• Added transformers to the shade plugin
  • manifest transformer: makes sure that our manifest.mf ends up in the final jar
  • apache license transformer: prevents apache licenses from ending up in our manifest
  • apache notice transformer: combines notices together into one NOTICE file

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• [N/A] I have tested this pull request for defects on a server.

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