Update Copyright across PennyLane

[PietropaoloFrisoni]

Context:

Description of the Change: We changed all the Copyrights in the comment section of each file to: Copyright 2018-2024 (...)

Benefits: Coherence and consistency.

Possible Drawbacks: None that I can think of.

Related GitHub Issues: None.

[github-actions]

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (215ab98) to head (031ef20).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6286   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files         444      444           
  Lines       42223    42223           
=======================================
  Hits        42100    42100           
  Misses        123      123           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.