Update C# language version to 10.0 to fix bug with .NET SDK 6 Preview 7 (#1541)

Our build started failing this morning because .NET SDK 6 moved from
Preview 6 to Preview 7 and its generated code now throws an error with
language version 9.0, but is allowed with 10.0.

Author: andyleejordan

PowerShellEditorServices.Common.props

@@ -4,7 +4,7 @@
     <VersionSuffix></VersionSuffix>
     <Company>Microsoft</Company>
     <Copyright>© Microsoft Corporation.</Copyright>
-    <LangVersion>9.0</LangVersion>
+    <LangVersion>10.0</LangVersion>
     <PackageTags>PowerShell;editor;development;language;debugging</PackageTags>
     <PackageLicenseUrl>https://raw.githubusercontent.com/PowerShell/PowerShellEditorServices/master/LICENSE</PackageLicenseUrl>
     <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>