chore: update to c# 9 (microsoft#1147)

* update to c# 9

* redundant LangVersion

* Update PlaywrightSharp.sln

Author: SimonCropp

Directory.Build.props

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project>
+  <PropertyGroup>
+    <LangVersion>9</LangVersion>
+  </PropertyGroup>
+</Project>

src/PlaywrightSharp.TestServer/PlaywrightSharp.TestServer.csproj

@@ -3,7 +3,6 @@
     <PropertyGroup>
         <TargetFrameworks>netcoreapp3.1;net5.0;net48</TargetFrameworks>
         <OutputType>Library</OutputType>
-        <LangVersion>8</LangVersion>
         <ReleaseVersion>0.0.0</ReleaseVersion>
     </PropertyGroup>
     <ItemGroup>

src/PlaywrightSharp.Tests/PlaywrightSharp.Tests.csproj

@@ -4,7 +4,6 @@
         <TargetFrameworks>netcoreapp3.1;net5.0;net48</TargetFrameworks>
         <IsPackable>false</IsPackable>
         <IsTestProject>true</IsTestProject>
-        <LangVersion>8.0</LangVersion>
         <ReleaseVersion>0.0.0</ReleaseVersion>
     </PropertyGroup>
     <Import Project="../Common/SignAssembly.props" />

src/PlaywrightSharp.sln

@@ -8,6 +8,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 	ProjectSection(SolutionItems) = preProject
 		.editorconfig = .editorconfig
 		Common\Dependencies.props = Common\Dependencies.props
+		..\Directory.Build.props = ..\Directory.Build.props
 		PlaywrightSharp.ruleset = PlaywrightSharp.ruleset
 		stylecop.json = stylecop.json
 		Common\SignAssembly.props = Common\SignAssembly.props

src/PlaywrightSharp/PlaywrightSharp.csproj

@@ -6,7 +6,6 @@
 		<Summary>Playwright-Sharp .NET library to automate Chromium, Firefox and WebKit with a single API.</Summary>
 		<Description>PlaywrightSharp is a .NET library to automate Chromium, Firefox and WebKit with a single API. Playwright is built to enable cross-browser web automation that is ever-green, capable, reliable and fast.</Description>
 		<TargetFrameworks>netstandard2.0;net5.0</TargetFrameworks>
-		<LangVersion>9.0</LangVersion>
 		<DebugType>embedded</DebugType>
 		<DebugSymbols>true</DebugSymbols>
 		<PublishRepositoryUrl>true</PublishRepositoryUrl>