Bump System.Collections.Immutable from 6.0.0 to 7.0.0 (#376)

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 6.0.0 to 7.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v6.0.0...v7.0.0)

---
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>

Directory.Packages.props

@@ -48,7 +48,7 @@
     <!-- Analyzers need to use older references to work in existing C# compilers. -->
     <PackageVersion Update="Microsoft.CodeAnalysis" Version="$(CodeAnalysisVersionForAnalyzers)" />
     <PackageVersion Update="Microsoft.CodeAnalysis.CSharp" Version="$(CodeAnalysisVersionForAnalyzers)" />
-    <PackageVersion Update="System.Collections.Immutable" Version="6.0.0" />
+    <PackageVersion Update="System.Collections.Immutable" Version="7.0.0" />
     <PackageVersion Update="System.Reflection.Metadata" Version="6.0.0" />
   </ItemGroup>
   <ItemGroup>