[Rust] upgrade dependency "flatbuffers" to 0.8

[asfimport]

flatbuffers 0.8.0 was released on Dec 10, 2020, with some notable changes:

verifier
common rust traits to FlatBufferBuilder
new VectorIter
add FlatBufferBuilder::force_defaults API
Optional Scalars
up to 2018 edition
possible performance speedup
... and minor breaking change to some APIs, for example: remove "get_", return Result.

Let's try this version.

Reporter: Qingyou Meng / @mqy
Assignee: Qingyou Meng / @mqy

Related issues:

• [Rust] upgrade dependency "flatbuffers" to release after 0.8 (is cloned by)

PRs and other links:

• GitHub Pull Request #8936

_{Note: This issue was originally created as ARROW-10938. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 8936
#8936