Bump System.Collections.Immutable from 6.0.0 to 7.0.0 (#7)

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](https://github.com/dotnet/runtime/commits)

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

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

Author: dependabot[bot]

src/MyNihongo.LinqAsync.csproj

@@ -20,7 +20,7 @@
 	</PropertyGroup>
 
 	<ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
-		<PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
+		<PackageReference Include="System.Collections.Immutable" Version="7.0.0" />
 	</ItemGroup>
 
 	<ItemGroup>