Bump System.Threading.Tasks.Extensions from 4.5.4 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-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj

@@ -23,7 +23,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'net48'">
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.6.3" />
   </ItemGroup>
 
   <ItemGroup>