fix backwards compatibility breakages from 1.21 api changes

[CyberFlameGO]

Information

This PR fixes #5851.

Details

Proposed fix:
Get the top inventory via reflection, and get the bottom inventory via getting clicked inventory rather than via view.

Environments tested:

Will test later on and populate these fields - just drafting the PR for review

OS:

Java version:

• [ ] Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• [ ] CraftBukkit/Spigot/Paper 1.12.2
• [ ] CraftBukkit 1.8.8

Demonstration:
N/A - if this PR will work then there is nothing to screenshot other than lack of exceptions in logs