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 Rework how Rust generated files are laid out #6731, undoing Change Rust generated file defaults #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. Rust verifier checks field alignment where C++ verifier does not #7015
• Rust soundness fixes #7518

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This issue was automatically closed due to no activity for 6 months plus the 14 day notice period.