minimum values for CL_DEVICE_HALF_FP_CONFIG should not be tied to an OpenCL version

[bashbaug]

The current minimum value for CL_DEVICE_HALF_FP_CONFIG has a dependency on OpenCL 2.0 or newer:

If half-precision is supported by the device, then the minimum half-precision floating-point capability for OpenCL 2.0 or newer devices is: [...]

I don't think this was intended and was introduced when the extensions were incorporated into the core specification. Note that there is no similar requirement in the last separate extension doc v3.0.15.

We should remove "for OpenCL 2.0 or newer devices", and check that there are no similar requirements for any other fp16 queries.