Pin the language version instead of building on preview (#839)

`preview` tracks whatever the installed SDK considers unreleased, so the
semantics of the code compiled here could change between SDK patch releases
with no signal in the repository. For a library that ships strong-named
binaries that is a dependency on a moving target rather than a feature.

C# projects pin to 14.0, which supplies everything preview did that this
repository uses, including extension members. F# does not accept a numeric
LangVersion of 14.0 -- FSC rejects it with FS0246 -- so the property is now
scoped by project extension and the F# wrappers take `latest`, the stable
equivalent of what they had.

The two analyzer projects still fail to build on Linux for an unrelated
reason (NETSDK1060 out of a backslash-separated ProjectReference); that is
unchanged by this commit and was verified against master.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Rafael-SOWNet

Sources/AngouriMath/AngouriMath.csproj

@@ -9,7 +9,7 @@
 
   <PropertyGroup>
     <OutputType>Library</OutputType>
-    <LangVersion>preview</LangVersion>
+    <LangVersion>14.0</LangVersion>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <PackageTags>$(PackageTags), csharp</PackageTags>
     <Description>Enables to work with formulas built in the code or from a string. Computing, derivating, latex rendering, compilation, solving equations and systems of equations analytycally, simplification, and much more. Read more on https://am.angouri.org.</Description>

Sources/Directory.Build.props

@@ -1,7 +1,14 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project>
   <PropertyGroup>
-    <LangVersion>preview</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
-</Project>
+
+  <PropertyGroup Condition="'$(MSBuildProjectExtension)' == '.csproj'">
+    <LangVersion>14.0</LangVersion>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(MSBuildProjectExtension)' == '.fsproj'">
+    <LangVersion>latest</LangVersion>
+  </PropertyGroup>
+</Project>

Sources/Samples/Samples/Playground.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>net10.0</TargetFramework>
-    <LangVersion>preview</LangVersion>
+    <LangVersion>14.0</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>