ARROW-7716: [Packaging][APT] Use the "main" component for Ubuntu 19.10
ARROW-7716: [Packaging][APT] Use the "main" component for Ubuntu 19.10

Because we always use the "main" component in
apache-arrow-archive-keyring.

Closes #6315 from kou/release-use-main-for-eoan and squashes the following commits:

a487d8d18 <Sutou Kouhei>  Use the "main" component for Ubuntu 19.10

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

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