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ARROW-12991: [CI] Migrate Travis-CI ARM job to "arm64-graviton2" arch

Closes #10478 from pitrou/ARROW-12991-arm64-graviton2

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