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GH-35987: [C++] Pin brew protobuf version to 21 (#36029)



### Rationale for this change

A temporary workaround for link errors caused by protobuf 23.

### What changes are included in this PR?

Pin brew protobuf version to 21 for now.

### Are these changes tested?

To be tested by CI pipelines.

### Are there any user-facing changes?

No

* Closes: #35987

Authored-by: Jin Shang <shangjin1997@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>