Pin Qiskit to <2 in next release (backport #904)

[mergify]

Summary

Pinning Qiskit<2 will avoid breaking changes to affect current installations. This potential effect was raised in #897. The constraint can be removed when QML achieves full compatibility with Qiskit 2.

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This is an automatic backport of pull request #904 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of de2cf2c has failed:

On branch mergify/bp/stable/0.8/pr-904
Your branch is up to date with 'origin/stable/0.8'.

You are currently cherry-picking commit de2cf2c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   constraints.txt

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   requirements.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally