Update to C# 14 and move suppressions to Directory.Build.props

- Update LangVersion from 13 to 14
- Remove NoWarn suppressions from individual csproj files
- Add WarningsNotAsErrors for NU1902, NU1903, CA1873 in Directory.Build.props
- This follows Microsoft's recommended approach for projects using TreatWarningsAsErrors

Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>

Author: Copilot

Directory.Build.props

@@ -1,14 +1,14 @@
 <Project>
   <PropertyGroup>
     <Nullable>enable</Nullable>
-    <LangVersion>13</LangVersion>
+    <LangVersion>14</LangVersion>
     <AnalysisMode>Recommended</AnalysisMode>
     <ImplicitUsings>enable</ImplicitUsings>
     <PreserveCompilationContext>true</PreserveCompilationContext>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <DockerDefaultTargetOS>Linux</DockerDefaultTargetOS>
-    <!-- CA1873: Suppress logging performance analyzer in .NET 10 for minor perf gains -->
-    <NoWarn>$(NoWarn);CA1873</NoWarn>
+    <!-- Exclude NuGet audit and analyzer warnings from being treated as errors -->
+    <WarningsNotAsErrors>$(WarningsNotAsErrors);NU1902;NU1903;CA1873</WarningsNotAsErrors>
 
     <!-- https://aka.ms/vs-build-acceleration -->
     <AccelerateBuildsInVisualStudio>True</AccelerateBuildsInVisualStudio>

EssentialCSharp.Web.Tests/EssentialCSharp.Web.Tests.csproj

@@ -7,9 +7,8 @@
     <IsPublishable>false</IsPublishable>
     <!--
     CA1707, Identifiers should not contain underscores - we allow these in test names
-    NU1902/NU1903, Suppress vulnerability warnings for transitive dependencies from design-time tools
     -->
-    <NoWarn>$(NoWarn);CA1707;NU1902;NU1903</NoWarn>
+    <NoWarn>$(NoWarn);CA1707</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>

EssentialCSharp.Web/EssentialCSharp.Web.csproj

@@ -1,8 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
   <PropertyGroup>
     <TargetFramework>net10.0</TargetFramework>
-    <!-- NU1902/NU1903, Suppress vulnerability warnings for transitive dependencies from design-time tools -->
-    <NoWarn>$(NoWarn);NU1902;NU1903</NoWarn>
   </PropertyGroup>
   <ItemGroup>
     <PlaceholderChapterOneHtmlFile Include="$(ProjectDir)/Placeholders/Chapters/01/Pages/*.html" />