Project non Version format of LangVersion fails to build

[ByronMayne]

Description:
There is currently an error that is generated if the project is targeting an older version of the language. It currently is this

 <Error
   Condition="$([MSBuild]::VersionGreaterThan('$(MinLangVersion)', '$(LangVersion)'))"
   Text="[SourceGenerator.Foundations] The project $(MSBuildProjectName) is currently using the language version $(LangVersion) which does not meet the minimum required of $(MinLangVersion). Read how to change it here https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version" />

The problem is there are validate values for Version that are not number, if this is used there is an build exception.

• preview
• latest
• latestMajor
• default
• ISO-2
• ISO-1

We should update the error to account for these values