move stable dep generation to its own cronjob workflow

[timmysilv]

Context: there are a lot of github notifs because of the dependency-tracking workflow, so it would be nice to reduce it.

Description of the Change: Run stable deps generation on a cronjob (or workflow_dispatch) instead of every merge to master (or PR in the case of docs)

Benefits:

• Less notifications!
• No more lightning sed hack needed (because I don't request latest lightning anymore). I removed the warning from the PR message too.
• I have confirmed for myself that the docs virtualenv is what I thought it was!

Possible Drawbacks:

• Not tied to successful builds, less data

Example job: https://github.com/PennyLaneAI/pennylane/actions/runs/8442938854/job/23125602801

[github-actions[bot]]

Hello. You may have forgotten to update the changelog! Please edit doc/releases/changelog-dev.md 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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.67%. Comparing base (57e8d93) to head (ff24cbd). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5440      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         414      414              
  Lines       38819    38527     -292     
==========================================
- Hits        38695    38402     -293     
- Misses        124      125       +1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[trbromley]

@timmysilv @Alex-Preciado @rashidnhm do we plan to proceed with this PR?

[timmysilv]

I would direct that question to you and the team! @albi3ro and @mudit2812 were discussing whether or not the per-PR updates were annoying, so I made this PR for us to have as an alternative. you may take it or leave it : )

[trbromley]

Thanks @timmysilv! I'll close this PR for now as it does not look like a priority, but we can always open if needed.