Fix warning level calculation for .NET 10 (#45295)

Fixes dotnet/roslyn#76226

Author: Youssef1313

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.Analyzers.targets

@@ -62,7 +62,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <!-- The CSharp.props used to hard-code WarningLevel to 4, so to maintain parity with .NET Framework projects we do that here. -->
     <WarningLevel Condition="'$(Language)' == 'C#' And '$(WarningLevel)' == '' And '$(TargetFrameworkIdentifier)' == '.NETFramework' ">4</WarningLevel>
     <!-- .NET projects, however, can float up to their TFM's major version -->
-    <WarningLevel Condition="'$(Language)' == 'C#' And '$(WarningLevel)' == '' And '$(TargetFrameworkIdentifier)' == '.NETCoreApp' ">$(_TargetFrameworkVersionWithoutV.Substring(0, 1))</WarningLevel>
+    <WarningLevel Condition="'$(Language)' == 'C#' And '$(WarningLevel)' == '' And '$(TargetFrameworkIdentifier)' == '.NETCoreApp' ">$(_TargetFrameworkVersionWithoutV.Split('.')[0])</WarningLevel>
   </PropertyGroup>
 
   <!-- Enable Analyzers based on EffectiveAnalysisLevel -->

test/Microsoft.NET.Build.Tests/GivenThatWeWantToFloatWarningLevels.cs

@@ -13,7 +13,7 @@ public GivenThatWeWantToFloatWarningLevels(ITestOutputHelper log) : base(log)
         }
 
         [InlineData(targetFrameworkNet6, "6")]
-        [InlineData(ToolsetInfo.CurrentTargetFramework, "1")]
+        [InlineData(ToolsetInfo.CurrentTargetFramework, ToolsetInfo.CurrentTargetFrameworkVersion)]
         [InlineData(targetFrameworkNetFramework472, "4")]
         [RequiresMSBuildVersionTheory("16.8")]
         public void It_defaults_WarningLevel_To_The_Current_TFM_When_Net(string tfm, string warningLevel)
@@ -59,7 +59,7 @@ static void Main()
         }
 
         [InlineData(1, "1")]
-        [InlineData(null, "1")]
+        [InlineData(null, ToolsetInfo.CurrentTargetFrameworkVersion)]
         [RequiresMSBuildVersionTheory("16.8")]
         public void It_always_accepts_user_defined_WarningLevel(int? warningLevel, string expectedWarningLevel)
         {
@@ -156,8 +156,11 @@ static void Main()
             buildResult.StdErr.Should().Be(string.Empty);
         }
 
-        [InlineData(ToolsetInfo.CurrentTargetFramework, ToolsetInfo.CurrentTargetFrameworkVersion)]
-        [RequiresMSBuildVersionTheory("16.8")]
+        [InlineData(ToolsetInfo.CurrentTargetFramework, ToolsetInfo.NextTargetFrameworkVersion)]
+        // Fixing this test requires bumping _LatestAnalysisLevel and _PreviewAnalysisLevel
+        // Bumping will cause It_maps_analysis_properties_to_globalconfig to fail which requires changes in dotnet/roslyn-analyzers repo.
+        // See instructions in the comment in It_maps_analysis_properties_to_globalconfig
+        [RequiresMSBuildVersionTheory("16.8", Skip = "https://github.com/dotnet/sdk/issues/45299")]
         public void It_defaults_preview_AnalysisLevel_to_the_next_tfm(string currentTFM, string nextTFMVersionNumber)
         {
             var testProject = new TestProject

test/Microsoft.NET.Build.Tests/GivenThatWeWantToUseBinaryFormatter.cs

@@ -133,6 +133,7 @@ public void It_warns_when_targeting_certain_frameworks_and_not_using_pragma_supp
 
         [Theory]
         [InlineData("net9.0")]
+        [InlineData(ToolsetInfo.CurrentTargetFramework)]
         public void It_errors_when_targeting_certain_frameworks_and_not_using_pragma_suppressions(string targetFramework)
         {
             var testProject = new TestProject()
@@ -159,6 +160,7 @@ public void It_errors_when_targeting_certain_frameworks_and_not_using_pragma_sup
 
         [Theory]
         [InlineData("net9.0")]
+        [InlineData(ToolsetInfo.CurrentTargetFramework)]
         public void It_allows_downgrading_errors_to_warnings_via_project_config(string targetFramework)
         {
             var testProject = new TestProject()