Bump pocketmine/pocketmine-mp from 5.6.0 to 5.8.1

Bumps [pocketmine/pocketmine-mp](https://github.com/pmmp/PocketMine-MP) from 5.6.0 to 5.8.1.
- [Release notes](https://github.com/pmmp/PocketMine-MP/releases)
- [Commits](pmmp/PocketMine-MP@5.6.0...5.8.1)

---
updated-dependencies:
- dependency-name: pocketmine/pocketmine-mp
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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