association attribute is obsolete

[dsoltesz]

generated code will throw hundreds of warnings

warning CS0618: 'AssociationAttribute' is obsolete: 'This attribute is no longer in use and will be ignored if applied.'

[Daniel-Svensson]

The code base for both Server and Client have the same issue.
It seems that .Net team unfortunately decided to obsolete the whole attribute just because it is not used på EF core, which is very sad.

My intial ideas to solve these are

• add temporary fix where warning "CS0618" is disabled in the generated code file (can be done using custom code generation using T4 templates)
• create a similar attribute "EntityAssociationAttribute" or similar that is used instead,
  • any solution needs to be backwards compatible so that AssociationAttribute on models are discovered

[Daniel-Svensson]

This is mostly solved by #509

[Daniel-Svensson]

• We want to change namespace to "System.ComponentModel.DataAnnotations" before release