Providing experiment annotation always override base class StartTime

[henrikt-ma]

This is a follow-up to https://github.com/modelica/ModelicaSpecification/pull/2999, since I'm not happy with my conclusion about the consequences in https://github.com/modelica/ModelicaSpecification/issues/2985#issuecomment-1822440449.

The problem is this part of the synopsis of experiment:

  Real StartTime(unit = "s") = 0;

because it means it is not possible to inherit StartTime from a base class.

In order to properly support inheritance of experiment options, I think we have to move the default value for StartTime out of the synopsis, and instead describe it in the semantics part.