Update dep version parsing

[joeycarter]

Context: At the end of a release cycle, it would be convenient to comment out the pennylane and lightning versions in the .dep-versions file, as so:

jax=0.4.28
...

# For a custom PL version, update the package version here and at
# 'doc/requirements.txt
# pennylane=0.41.0-dev1

# For a custom LQ/LK version, update the package version here and at
# 'doc/requirements.txt'
# lightning=0.41.0-dev1

This allows keeping the correct format in place without explicitly pinning the package version. However, comments are not currently recognized when the .dep-versions file is parsed in setup.py.

Description of the Change: Replace the old file parsing, which performed simple string splitting based on the number of characters after jax=, pennylane= and lightning=, with more robust regex-based parsing that also allows for lines to be commented out.

The pennylane and lightning versions introduced at the end of the PennyLane/Lightning 0.40.0 release cycle have also been commented out.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md on your branch with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[joeycarter]

There are failures using the Lightning Kokkos backend, I think because the CI action is pinning PennyLane-Lightning-Kokkos==0.39.0. We can try bumping this to 0.40.0 and see if this fixes the issue. I've opened a separate PR for this: #1462.

[dime10]

Thank you 🙏