Breaking changes in Rust for 3.0.0 release

[CasperN]

The main release tracker is at #6636, and this issue is just for Rust and its breaking changes.

• [ ] Stop generating code with #deperacated markers, I promised they'd go away in 2021, but here we are :)
• [ ] Make the reworked generated file format the default #6731, undoing #7101. I'd prefer to delete the old way entirely but perhaps there'll be a need to retain legacy behavior behind a flag?
• [ ] Rust should use Flatbuffer's notion of alignment, not rustc's. #7015
• [x] #7518