fix(build): add netstandard2.0 target to eliminate System.Runtime version mismatch

The net9.0 DLL references System.Runtime 9.0.0.0 which is unavailable
on GitHub runners where pwsh ships with .NET 8.x. Adding netstandard2.0
as a target produces a DLL compatible with any .NET version (PS 5.1 on
.NET Framework through PS 7.x on .NET 8/9).

- Add netstandard2.0 to TargetFrameworks in Core and module csproj
- Update #if NET48 conditionals to include NETSTANDARD2_0
- Switch CI workflows from net9.0 to netstandard2.0 for Pester tests
- Update _TestHelper.ps1 framework search order

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/PSProxmoxVE.Tests/_TestHelper.ps1

@@ -30,10 +30,10 @@ $repoRoot = Resolve-Path (Join-Path $PSScriptRoot '../..')
 $moduleRoot = Join-Path $repoRoot 'src/PSProxmoxVE'
 
 if ($PSVersionTable.PSEdition -eq 'Core') {
-    $frameworks = @('net9.0', 'net48')
+    $frameworks = @('netstandard2.0', 'net9.0', 'net48')
 }
 else {
-    $frameworks = @('net48', 'net9.0')
+    $frameworks = @('net48', 'netstandard2.0', 'net9.0')
 }
 
 $searchPaths = foreach ($fw in $frameworks) {