Inconsistencies between SDF tests, SDF docs, and sdformat/sdf/**/*.sdf files

I apologize for the slightly generic title. I figured that I will create this summative issue instead of creating a new issue for each problem to avoid flooding the issue list (any more than I already have).

There are multiple places where the sdf files used for CI, the SDF spec, and the sdf used to generate XSD bindings disagree. The problem with that is that I can't resolve these issues myself; I don't know which of these is authorative. Likely it should be decided on a case-by-case basis; however, I don't have the authority to make this decision.

Here is the list of mismatches that I am aware currently off:

1. [x] SDF (all versions) `/link/sensor` specifies `@required=0` (meaning 0 or 1 occurrences iirc), but the test `world_with_state.sdf` (v1.6 SDF) (which uses a PR2 model) defines two different sensors in several `.//link`s. I would change `@required=*` in the next SDF version, and remove the additional sensor elements from the test (since 1.6 doesn't allow multiple sensors). The same problem as above occurs in `sensors.sdf`, which appears to specify every possible sensor inside a single link element.. sensors.sdf is SDF v1.7 **Note**: This is fixed [in the scikit-bot schema](https://github.com/FirefoxMetzger/scikit-bot/tree/main/skbot/ignition/sdformat/schema) but not yet fixed in the spec.
3. [x] `sdformat/sdf/**/schema/types.xsd` defines `` as a restriction on `xsd:double`; however, the spec specifies it as `[{seconds} {nanoseconds}]` and /**/state.sdf specify uses the default `"0 0"`. This is a duplicate of https://github.com/ignitionrobotics/sdformat/issues/642