build(deps): bump System.Collections.Immutable in /src (#816)

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v5.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: System.Collections.Immutable
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/Splat.Exceptionless/Splat.Exceptionless.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Exceptionless" Version="4.6.2" />
-    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
 
   <ItemGroup>

src/Splat.Microsoft.Extensions.Logging/Splat.Microsoft.Extensions.Logging.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Logging" Version="5.0.0" />
-    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Splat\Splat.csproj" />

src/Splat.NLog/Splat.NLog.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="NLog" Version="4.7.12" />
-    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Splat\Splat.csproj" />

src/Splat.Serilog/Splat.Serilog.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Serilog" Version="2.10.0" />
-    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Splat\Splat.csproj" />