Bump Backport.System.Threading.Lock to 3.1.5

Author: MarkCiliaVincenti

BitFaster.Caching/BitFaster.Caching.csproj

@@ -54,7 +54,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" PrivateAssets="All" />
-    <PackageReference Include="Backport.System.Threading.Lock" Version="3.1.4">
+    <PackageReference Include="Backport.System.Threading.Lock" Version="3.1.5">
        <PrivateAssets>all</PrivateAssets>
        <IncludeAssets>analyzers</IncludeAssets>
     </PackageReference>