[Rust] [Arrow] catch up with flatbuffers 0.8.1

[asfimport]

The major change in https://docs.rs/flatbuffers/0.8.1/flatbuffers/index.html is PR https://github.com/google/flatbuffers/pull/6393,,) which fixed some possible memory alignment issues.

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

PRs and other links:

• GitHub Pull Request #9176

_{Note: This issue was originally created as ARROW-11222. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 9176
#9176