Fix failing CI, local dev dockerfile setup, implement Centos 6 deprecation

[alopezz]

What does this PR do?

• Implements deprecation of Centos <7 (cf. Limit the versions of the agent that can be installed on Centos < 7 ansible-datadog#556, etc.).
• Makes the provided Dockerfile build successfully and capable of running kitchen-docker tests locally (at least on Intel).
• Updates documentation related to that setup
• Updates machine image used for kitchen-docker-tests job, since that fixes some failing tests (example).

Motivation

BARX-282

Additional Notes

• I tested the "erroring out on invalid pinned version" manually because I don't feel too confident on implementing those tests myself at this point.
• I'd have rather split this in several PR's into several since it's tackling a few concerns at the same time, but I had no choice if I wanted to test things locally and the CI green.
• Tests seem to work fine on Apple silicon (M1s, etc.), though they're expected to run more slowly due to the extra emulation layer.

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

Reviewer's Checklist

[Pythyu]

LGTM just need to wait that the docker kitchen tests still works before merge

[jhgilbert]

Approved with a minor suggestion, thanks!

Significant changes since this review

[Pythyu]

Aside from my single comment LGTM 🚀