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GH-35961: [C++][FlightSQL] Accept Protobuf 3.12.0 or later (#35962)

### Rationale for this change

We can use `optional` with Protobuf 3.12.0 by specifying `--experimental_allow_proto3_optional` explicitly.

If we accept Protobuf 3.12.0 or later for Flight SQL, we can use system Protobuf on Ubuntu 22.04. Because Ubuntu 22.04 ships Protobuf 3.12.4.

### What changes are included in this PR?

Specify  `--experimental_allow_proto3_optional` explicitly when Protobuf is < 3.15.0. 

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #35961

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>