RUSTSEC-2021-0122: Generated code can read and write out of bounds in safe code

[github-actions]

Generated code can read and write out of bounds in safe code

Details
Package	flatbuffers
Version	2.1.2
URL	google/flatbuffers#6627
Date	2021-10-31

Code generated by flatbuffers' compiler is unsafe but not marked as such.
See google/flatbuffers#6627 for details.

For example, if generated code is used to decode malformed or untrusted input,
undefined behavior (and thus security vulnerabilities) is possible even without
the use of the unsafe keyword, violating the the meaning of "safe" code;

All users that use generated code by flatbuffers compiler are recommended to:

1. not expose flatbuffer generated code as part of their public APIs
2. audit their code and look for any usage of follow, push, or any method that uses them
   (e.g. self_follow).
3. Carefuly go through the crates' documentation to understand which "safe" APIs are not
   intended to be used.

See advisory page for additional details.

[houqp]

transient dependency