[Packaging][Ubuntu] Drop support for 20.10

[asfimport]

It reached EOL.

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #10881

_{Note: This issue was originally created as ARROW-13565. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 10881
#10881