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ARROW-17205: [Dev][Release] Merge script should prompt for next version when maintenance branch is created (#13708)

Currently the merge_pr script is asking to merge to 9.0.0 when the fetature freeze has been implemented. If the maintenance branch is created we should filter them from the possible versions.

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