Bump System.Threading.Tasks.Extensions from 4.6.0 to 4.6.3

---
updated-dependencies:
- dependency-name: System.Threading.Tasks.Extensions
  dependency-version: 4.6.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

MailKit/MailKitLite.csproj

@@ -57,7 +57,7 @@
   </PropertyGroup>
 
   <ItemGroup Condition=" $(TargetFramework.StartsWith('net4')) Or '$(TargetFramework)' == 'netstandard2.0' ">
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.6.0" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.6.3" />
   </ItemGroup>
 
   <ItemGroup Condition=" $(TargetFramework.StartsWith('net4')) Or $(TargetFramework.StartsWith('netstandard')) Or '$(TargetFramework)' == 'net8.0' ">