deprecate LeafletFinder and schedule for removal in 3.0

[orbeckst]

The original LeafletFinder algorithm lacks a bit in robustness (#2843) and is also fairly specialized. It also adds networkx as a MDAnalysis dependency. Similar algorithms are available in the MDAKit lipyds.

In line with discussions at the UGM (roadmap for 3.0), we should therefore

• [ ] deprecate LeafletFinder (and point to lipyds's https://lipyds.readthedocs.io/en/latest/leafletfinder.html)
• [ ] #4544

[orbeckst]

@fiona-naughton was this on your April TODO list?

[orbeckst]

Alternatively, is this something that @lilyminium would want to take on, given that we are sending people to lipyds for the alternative?