build(deps): bump Microsoft.ApplicationInsights in /src

Bumps [Microsoft.ApplicationInsights](https://github.com/Microsoft/ApplicationInsights-dotnet) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/Microsoft/ApplicationInsights-dotnet/releases)
- [Changelog](https://github.com/microsoft/ApplicationInsights-dotnet/blob/main/CHANGELOG.md)
- [Commits](microsoft/ApplicationInsights-dotnet@2.20.0...2.21.0)

---
updated-dependencies:
- dependency-name: Microsoft.ApplicationInsights
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

src/Splat.ApplicationInsights/Splat.ApplicationInsights.csproj

@@ -11,7 +11,7 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.ApplicationInsights" Version="2.20.0" />
+    <PackageReference Include="Microsoft.ApplicationInsights" Version="2.21.0" />
   </ItemGroup>
 
   <ItemGroup>