Define and use constants based on EF Core version, not dotnet one

[Akridian]

EF Core 9 can be used with dotnet 8 (as we do in our projects). And I believe EF Core 8 is suitable for dotnet 9.

Current configuration leads to incorrect behaviour when EntityFrameworkCore.UseRowNumberForPaging is used with dotnet 8 and EF Core 9.

This PR introduces new properties and compilation constants. With them use can publish two version of the package: one for EF Core 8 and the other for EF Core 9.

According to docs -Properties UseEfCore8=false -Properties UseEfCore9=true should do it. Just flip around boolean flags.

I also made Version property dependant on those flags, but it is just a suggestion.