Improve GetAllValidNETPreprocessorSymbols

This works, but I think it is cleaner, to just write out the individual `yield return` lines.

_Originally posted by @Keboo in https://github.com/IntelliTect/EssentialCSharp/pull/584#discussion_r1383683019_

Maybe something like:
```C#
switch
(
(majorVersion, minorVersion)
)
{
case (5, 0):
yield return "NET5_0_OR_GREATER";
yield return "NET5_0";
goto case (3, 1);
case (3, 1):
yield return "NETCOREAPP3_1_OR_GREATER";
yield return "NETCOREAPP3_1";
goto case (3, 0);
case (3, 0):
yield return "NETCOREAPP3_0_OR_GREATER";
goto case (2, 1);
case (2, 1):
yield return "NETCOREAPP2_1_OR_GREATER";
goto case (2, 0);
case (2, 0):
yield return "NETCOREAPP2_0_OR_GREATER";
goto case (1, 1);
case (1, 1):
yield return "NETCOREAPP1_1_OR_GREATER";
goto case (1, 0);
case (1, 0):
yield return "NETCOREAPP1_0_OR_GREATER";
break;
default:
break;
```
This needs more cleanup and work, but general structure is there

Locate the GetAllValidNETPreprocessorSymbols entry point and compare its current implementation with the switch structure suggested in the issue. Clean up the implementation while preserving the expected symbols for each supported version, then verify the project still builds and its existing checks pass.

Scritto dal modello di indicizzazione a partire dal testo della issue.