Using .net 8. Set warnings for .net 8.

Analyzers/Directory.Build.props

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <Nullable>enable</Nullable>
     <AnalysisLevel>latest-recommended</AnalysisLevel>
-    <WarningLevel>7</WarningLevel>
+    <WarningLevel>8</WarningLevel>
     <CharacterSet>UTF-8</CharacterSet>
     <Deterministic>true</Deterministic>
     <UTF8OutPut>true</UTF8OutPut>