Update to MTK@v10 and simplify connectors

[SebastianM-C]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I've taken the opportunity to use this breaking change in MTK to also have a breaking change here and get rid of the MTKStdLib dependency. This should allow people to use the SymbolicNeuralNetwork interface without that extra dependency and we didn't really need the RealInput/OutputArray blocks. The interface ended up being a bit awkward and when the block interacts with the rest of the model you will most likely want to use it across several equations, so the input/output blocks don't bring any advantage, it mostly make the code more complex.

As such, I changed the inputs and outputs to normal vector variables and we can now also connect them to RealInput/OutputArray blocks if we use the inner .us.

[SebastianM-C]

Thanks, @ChrisRackauckas
I still need to do the docs update, but the package code should be ready (hence the draft status).

[SebastianM-C]

This should be ready now, tests on release and lts pass and the docs build. I want to do a larger documentation rewrite, but that's a separate PR.

The formatting and invalidations tests might be broken, the formatter is happy locally.