Rename "qubit" references to device element where appropriate

Explanation of changes

The references (variable names, keys, docstrings) to "qubits" are renamed to "device element" where it makes sense. This is supposed to be a backwards compatible change (no schedule, no serialized object needs to be modified).

Motivation of changes

In some cases the qubit is not the same as the device element (particularly in spin qubits). The code always assumes that they're the same, but this is misleading. In a lot of cases it is more appropriate to refer to "device element" rather than qubit. Note, that this change does not replace "qubit" with "device element" everywhere, because in some documentation or other places, the word qubit is more appropriate.

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed (in-depth by a knowledgeable contributor), and is approved by a project maintainer.
• New code is covered by unit tests (or N/A).
• New code is documented and docstrings use numpydoc format (or N/A).
• New functionality: considered making private instead of extending public API (or N/A).
• Public API changed: added @deprecated and entry in deprecated code suggestions (or N/A).
• Newly added/adjusted documentation and docstrings render properly (or N/A).
• Pipeline fix or dependency update: post in #software-for-developers channel to merge main back in or update local packages (or N/A).
• Tested on hardware (or N/A).
• CHANGELOG.md for breaking changes and AUTHORS.md have been updated (or N/A).
• Update Hardware backends documentation if backend interface change or N/A
• Check whether performance is significantly affected by looking at the Performance metrics results.
• Windows tests in CI pipeline pass (manually triggered by maintainers before merging).
  • Maintainers do not hit Auto-merge, we need to actively check as manual tests do not block pipeline

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