[C++] Upgrade to Flatbuffers 1.12

[asfimport]

This will pick up a UBSan fix in google/flatbuffers#5355

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #6678

_{Note: This issue was originally created as ARROW-8178. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 6678
#6678