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ARROW-16349: [Release][Packaging][RPM] Remove ed25519 keys from KEYS

It seems that ed25519 isn't supported on Red Hat Enterprise Linux 8
or earlier.

Closes #13002 from kou/packaging-rpm-keys

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