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ARROW-16731: [C++] Bump version of vendored mimalloc (#13295)

The 2.0 line should probably be stable enough now.

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>