[C++] Bump Protobuf version to 3.15 when Flight is enabled

[asfimport]

Try bumping Protobuf to 3.15 so that FlightSQL can use the new optional feature.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

PRs and other links:

• GitHub Pull Request #11006

_{Note: This issue was originally created as ARROW-13760. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 11006
#11006