remove centos7 and buster

[braydonk]

Description

CentOS 7 and Debian 10 Buster are EOL. Remove them from the project spec.

Related issue

b/350698170 and b/350698983

How has this been tested?

Integration tests in this PR.

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.

[igorpeshansky]

LGTM

[braydonk]

I guess I opened too many PRs today and exhausted the quota. The tests that have quota problems should not be at all affected by this cleanup, so I am going to merge.