[release/6.0] Test on CentOS 7, not RedHat 7

[dougbu]

• https://github.com/dotnet/dnceng/issues/4298 part 1/II
  • contributes to https://github.com/dotnet/dnceng/issues/3189
• use Docker container for CentOS, mimicking https://github.com/dotnet/runtime/pull/105063

To double check:

• [x] The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md