Update purity exception for Streams.print

[henrikt-ma]

In https://specification.modelica.org/master/functions.html#pure-modelica-functions there is the following exception for print:

A deprecated semantics is that external functions (and functions defined in Modelica directly or indirectly calling them) without pure or impure keyword are assumed to be impure, but without any restriction on calling them. Except for the function Modelica.Utilities.Streams.print, a diagnostic must be given if called in a simulation model.

The problem is that Modelica.Utilities.Streams.print is nowadays explicitly marked impure, so the exception is only causing confusion at the moment.

This PR updates the specification so that it applies to the current MSL. The most important part, that there is a loophole for calling print anywhere, still applies to older MSL where print is not explicitly declared impure. The only change for older MSL is that the deprecated semantics demands a diagnostic in this case – I find it better that tools act on their own on this matter than to elaborate the rules for diagnostics in the specification.