Remove OCI manifest list limitation

[honnix]

It seems this is no longer a limitation and the latest version of the plugin supports it well.

Thank you for your interest in contributing! For general guidelines, please refer to
the contributing guide.

Before filing a pull request, make sure to do the following:

• Create a new issue at https://github.com/GoogleContainerTools/jib/issues/new/choose.
• Ensure that your implementation plan is approved by the team.
• Verify that integration tests and unit tests are passing after the change.
• Address all checkstyle issues. Refer to
  the style guide.

This helps to reduce the chance of having a pull request rejected.

Fixes #<issue_number_goes_here> 🛠️

[honnix]

Ah, maybe this means not supporting creating OCI manifest list, as described by #2748.