Modernize CI build and bump net6.0 targets to net8.0

.NET 6 is end-of-support and was removed from the hosted windows-latest
image, so the pipeline could no longer build or test. Bump the two test
projects and WindowsImaging from net6.0 to net8.0 (LTS) and pin the SDK
with an explicit UseDotNet@2 step.

Replace the deprecated UseGitVersion@5 task with the GitVersion 6
setup/execute task pair. The existing GitVersion.yml validates and runs
cleanly under GitVersion 6, so it is left unchanged.

Author: fw2568

azure-pipelines.yml

@@ -15,9 +15,19 @@ variables:
   buildConfiguration: 'Release'
 
 steps:
-- task: UseGitVersion@5
+- task: UseDotNet@2
+  displayName: install .NET SDK
   inputs:
-    versionSpec: '5.x'
+    packageType: sdk
+    version: '8.0.x'
+
+- task: gitversion/setup@3
+  displayName: install GitVersion
+  inputs:
+    versionSpec: '6.x'
+
+- task: gitversion/execute@3
+  displayName: determine version
 
 - task: DotNetCoreCLI@2
   inputs:

src/CloudInit.ConfigDrive.WindowsImaging/CloudInit.ConfigDrive.WindowsImaging.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net6.0-windows</TargetFrameworks>
+    <TargetFrameworks>net8.0-windows</TargetFrameworks>
     <AssemblyName>Dbosoft.CloudInit.ConfigDrive.WindowsImaging</AssemblyName>
     <RootNamespace>Dbosoft.CloudInit.ConfigDrive</RootNamespace>
     <Nullable>enable</Nullable>

test/CloudInit.ConfigDrive.Core.Test/CloudInit.ConfigDrive.Core.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net6.0</TargetFrameworks>
+    <TargetFrameworks>net8.0</TargetFrameworks>
      <IsPackable>false</IsPackable>
   </PropertyGroup>
 

test/CloudInit.ConfigDrive.NoCloud.Test/CloudInit.ConfigDrive.Core.NoCloud.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net6.0</TargetFrameworks>
+    <TargetFrameworks>net8.0</TargetFrameworks>
      <IsPackable>false</IsPackable>
   </PropertyGroup>