Upgrade bitflags dependency in Rust implementation to version 2.6.0

[gschulze]

The bitflags dependency in the Rust implementation has version 1.2.1, which is about five years old. When using flatbuffers with other crates that depend on a 2.x version of bitflags, this leads to duplicated dependencies. This could be avoided by updating the bitflags dependency to version 2.6.0.

[aardappel]

This will need to be reattempted, see: #8411

[XinyuZeng]

Any updates? This is the same for the num_enum dependency. Using the same 24.12.23 version for flatbuffer and flexbuffer, they result in duplicate dependency because of this, which is weird.