Add stable-2.15 test jobs for collections

2.15 release dates are given here: https://issues.redhat.com/browse/AAP-5652

2.15 feature freeze and branching from devel branch is happening between 3rd-7th April which means we need to add 2.15 test job in our collections CI next week. 

Definition of Done:

• 2.15 integration test job
  • https://github.com/ansible/ansible-zuul-jobs/blob/master/zuul.d/cisco-nxos-jobs.yaml#L73
• 2.15 unit test job
  • https://github.com/ansible-network/github_actions/blob/main/.github/workflows/unit_galaxy.yml
  • https://github.com/ansible-network/github_actions/blob/main/.github/workflows/unit_source.yml
• 2.15 sanity test job
  • https://github.com/ansible-network/github_actions/blob/main/.github/workflows/sanity.yml