Bump System.Collections.Immutable from 1.5.0 to 8.0.0

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 1.5.0 to 8.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](https://github.com/dotnet/runtime/commits/v8.0.0)

---
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>

Tests/FluentAssertions.Equivalency.Specs/FluentAssertions.Equivalency.Specs.csproj

@@ -17,7 +17,7 @@
       FluentAssertions.Specs.csproj for net47 does not generate a binding redirect from 4.0.4.0 to 4.0.4.1
       Upgrading to 1.6.0 gives "Could not load file or assembly 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.0"
     -->
-    <PackageReference Include="System.Collections.Immutable" Version="[1.5.0]" />
+    <PackageReference Include="System.Collections.Immutable" Version="[8.0.0]" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'net6.0' OR '$(TargetFramework)' == 'net8.0'">

Tests/FluentAssertions.Specs/FluentAssertions.Specs.csproj

@@ -17,7 +17,7 @@
       FluentAssertions.Specs.csproj for net47 does not generate a binding redirect from 4.0.4.0 to 4.0.4.1
       Upgrading to 1.6.0 gives "Could not load file or assembly 'System.Runtime.CompilerServices.Unsafe, Version=4.0.4.0"
     -->
-    <PackageReference Include="System.Collections.Immutable" Version="[1.5.0]" />
+    <PackageReference Include="System.Collections.Immutable" Version="[8.0.0]" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'net6.0' OR '$(TargetFramework)' == 'net8.0'">