chore: Replace nginxinc GH org mentions with nginx

[alessfg]

Proposed changes

See title. Not every mention has been changed since:
a) The Ansible project board still needs to be migrated
b) The roles are still under the nginxinc org in Ansible Galaxy

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added Molecule tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant Molecule tests pass after adding my changes.
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md).