replaced CanWrite with GetSetMethod != null

[DaniD3v]

CanWrite returns true if the property has a set accessor,
even if the accessor is private, internal (or Friend in Visual Basic), or protected.

Source

This is a problem if someone were to write their Property like this:

public decimal Limit {get; private set;}

canWrite will return true here even tho you can't actually write to it.

This PR only fixes a single occasion of this in 05_Vererbung but a single rg CanWrite shows that this anti-pattern is used across the whole teaching material

PLF/Types - Plf3bhif20191113/Program.cs:            score += AssertTrue(() => info.CanWrite == false);
PLF/Types - Plf3bhif20191113/Program.cs:            score += AssertTrue(() => info2.CanWrite == false);
01 Types/03_Properties.md:            if (typeof(Teacher).GetProperty(nameof(Teacher.Firstname))?.CanWrite == false
01 Types/03_Properties.md:                && typeof(Teacher).GetProperty(nameof(Teacher.Lastname))?.CanWrite == false)
01 Types/03_Properties.md:            if (typeof(Teacher).GetProperty(nameof(Teacher.Longname))?.CanWrite == false
01 Types/03_Properties.md:            if (typeof(Teacher).GetProperty(nameof(Teacher.Shortname))?.CanWrite == false
01 Types/03_Properties.md:            if (typeof(Teacher).GetProperty(nameof(Teacher.IsSchoolEmail))?.CanWrite == false
01 Types/Uebungen/Blog/BlogManager.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);
01 Types/Uebungen/LibraryManager/LibraryManager.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);
01 Types/Uebungen/Anmeldesystem/RegistrationSystem.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);
01 Types/Uebungen/Anmeldesystem/RegistrationSystem.Application/Program.cs:                () => typeof(RegistrationService).GetProperties().Any() && typeof(RegistrationService).GetProperties().All(p => p.CanWrite == false),
01 Types/Uebungen/Anmeldesystem/RegistrationSystem.Application/Program.cs:                () => typeof(Applicant).GetProperties().Any() && typeof(Applicant).GetProperties().All(p => p.CanWrite == false),
01 Types/Uebungen/Anmeldesystem/RegistrationSystem.Application/Program.cs:                () => typeof(AcceptedApplicant).GetProperties().Any() && typeof(AcceptedApplicant).GetProperties().All(p => p.CanWrite == false),
01 Types/Uebungen/Kalender/TeamCalendar.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);
01 Types/Uebungen/HealthChecker/HealthChecker.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);
01 Types/Uebungen/HealthChecker/HealthChecker.Application/Program.cs:                CheckAndWrite(() => typeof(Vaccination).GetProperty(nameof(Vaccination.FirstVaccination))?.CanWrite == false, "FirstVaccination is read-only.");
01 Types/Uebungen/CarRentalService/CarRentalService.Application/TestHelpers.cs:            type.GetProperty(propertyName)?.CanWrite == false;
01 Types/Uebungen/CarRentalService/CarRentalService.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);
02 Linq/02_Filterung/LinqUebung1/LinqUebung1.Application/TestHelpers.cs:            return type.GetProperties().All(p => p.CanWrite == false);

In my commit I also added a method to make the code more DRY.
I don't really get why this isn't done in all tests because those should follow good practices in case students were to learn from them.