checkid vs id parameter issue

Apparently there is some confusion around registering a check. This issue collects the data, issues and PRs around that so that we can decide what to do.

* https://github.com/hashicorp/consul/pull/7393
* https://github.com/hashicorp/consul/pull/7352
* https://github.com/hashicorp/consul/issues/6635
* https://github.com/hashicorp/consul/pull/7364
* #7733

Maybe related:

* https://github.com/hashicorp/consul/issues/6923

Possible root causes:

* https://github.com/hashicorp/consul/pull/6874

Action items:
* [ ] fix the bug in `consul services register` documented in the [comment below](https://github.com/hashicorp/consul/issues/7566#issuecomment-622513800)
* [ ] consider making the upgrade from pre-1.7.x smoother by restoring support for both field names (CheckID and ID). This would also make the request/response pairs use the same name, where as currently they are different.