Restore qiskit <1.0 bound prior to release

[wshanks]

This issue is opened assuming #1323 removes the qiskit<1.0 constraint from the qiskit-experiments requirements on main. The constraint should be restored on the 0.6 release branch prior to release (unless we decide we do not need the constraint).

[coruscating]

With our current release workflow, we need to restore the <1.0 bound in the last PR to main before release and then remove it in the post-release PR which bumps the package version string.

[wshanks]

Closing because the Qiskit 1.0 release candidate is out and the qiskit-experiments main branch should be compatible with it now.