[sc-73935]

* Remove deprecated code from demos.
* Update various `from pennylane import numpy as np` to `import numpy as
np`.

**Summary:**
Fixes a merge conflict between `master` and `dev` introduced by #1232
(due to divergent `dateOfLastModification`).

To reproduce this PR:
1. Run `git checkout dev`.
2. Run `git checkout -b merge-master-into-dev`.
3. Run `git merge master`.
4. Accept all incoming changes for merge conflicts on
`dateOfLastModification`.

**Relevant GHA Workflow Runs:**
* https://github.com/PennyLaneAI/qml/actions/runs/11241666164
* https://github.com/PennyLaneAI/qml/actions/runs/11253088983

---------

Co-authored-by: David Wierichs <davidwierichs@gmail.com>
Co-authored-by: Korbinian Kottmann <43949391+Qottmann@users.noreply.github.com>
Co-authored-by: Ivana Kurečić <ivana@xanadu.ai>

- Branched off dev
- Merged Master in via `--allow-unrelated-histories`

Co-authored-by: Jack Brown <jack.brown486@gmail.com>

Steps used for this merge:

1. Create new branch from `dev`
2. Merge `master` into that branch
3. Open PR

---------

Co-authored-by: Jack Brown <jack.brown486@gmail.com>
Co-authored-by: Ivana Kurečić <ivana@xanadu.ai>

Steps used:
* Checkout new branch from `dev`
* Merge `master` using `git merge master`
* Publish branch and open PR

---------

Co-authored-by: Jack Brown <jack.brown486@gmail.com>
Co-authored-by: Ivana Kurečić <ivana@xanadu.ai>
Co-authored-by: bellekaplan <167111433+bellekaplan@users.noreply.github.com>

…m/PennyLaneAI/qml into use_jax_qchem_demos_requires_grad