Update dependency JetBrains.Annotations to v2021.3.0 (#140)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

Roslyn.Testing/Roslyn.Testing.csproj

@@ -6,7 +6,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-      <PackageReference Include="JetBrains.Annotations" Version="2021.2.0" />
+      <PackageReference Include="JetBrains.Annotations" Version="2021.3.0" />
       <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="4.0.1" />
       <PackageReference Include="Shouldly" Version="4.0.3" />
     </ItemGroup>

System.IO.Abstractions.Analyzers/System.IO.Abstractions.Analyzers.csproj

@@ -30,7 +30,7 @@
 	  <Optimize>false</Optimize>
 	</PropertyGroup>
 	<ItemGroup>
-		<PackageReference Include="JetBrains.Annotations" Version="2021.2.0" PrivateAssets="all" />
+		<PackageReference Include="JetBrains.Annotations" Version="2021.3.0" PrivateAssets="all" />
 		<PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.3.3" PrivateAssets="all" />
 		<PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="4.0.1" PrivateAssets="all" />
 		<PackageReference Update="NETStandard.Library" PrivateAssets="all" />