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ARROW-14747: [Release] Add a script to merge changes in release branch

https://cwiki.apache.org/confluence/display/ARROW/Release+Management+Guide#ReleaseManagementGuide-Post-releasetasks

    1.  [ ] a) open a pull request to bump the version numbers in the
               source code on the development branch (`master` in case of major
               releases and `maint-<version>` for minor and patch releases)

            b) create and push a development tag
               `apache-arrow-<new-version>.dev` to the same development
               branch pointing to the merge commit of the pull request from
               the previous step

Closes #11733 from kou/release-post-merge

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