[Packaging][CentOS] Drop support for CentOS 8

[asfimport]

Because it'll reach EOL at 2021-12.

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

PRs and other links:

• GitHub Pull Request #11909

_{Note: This issue was originally created as ARROW-15038. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 11909
#11909