Start keeping stable branches after releases

[ElePT]

What is the expected enhancement?

I would like to propose using stable branches to isolate released code. This would make the branch structure consistent with qiskit, qiskit-ibm-runtime and other relevant ecosystem projects, as well as add the following advantages:

• Patch Management: After tagging v0.27.0 on main, create stable/0.27 so you can release v0.27.1, v0.27.2 without pulling in new features.

• Multiple Maintained Versions: If you later release v0.28.0, you can still patch 0.27.x from its own branch.

• Industry Standard for Qiskit and beyond: Most mature projects (Django, Kubernetes...) use stable branches for predictable maintenance and security fixes.

Stable branches isolate bug fixes from ongoing development, reduce risk, and make patch releases fast and clean.