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GH-40209: [C++][CMake] Use "RapidJSON" CMake target for RapidJSON (#40210)

### Rationale for this change

Because upstream uses "RapidJSON" for CMake target name.

### What changes are included in this PR?

Rename "rapidjson::rapidjson" to "RapidJSON".

FindRapidJSONAlt.cmake provides "RapidJSON" CMake target instead of just providing RAPIDJSON_INCLUDE_DIR.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #40209

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