make warnings into errors

[timmysilv]

Context:
Warnings-as-errors is great, and we should enable it.

Description of the Change:
Set all warnings as errors, except for all the warnings that exist today

Benefits:
Any new warnings will immediately be caught and converted into errors

Possible Drawbacks:
To make this extra-useful, someone ought to work on reducing this list by addressing the warnings. At least this makes it more concretely actionable:

1. remove an ignore filter line from this file
2. make a PR
3. choose one:
   a. fix the now-failing tests by fixing the root cause
   b. filter the warnings within the now-failing tests
   c. put back the filter and enshrine it forevermore as a PennyLane standard
4. repeat

[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 99.63%. Comparing base (0a0e208) to head (e206560).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6641   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files         468      468           
  Lines       44132    44132           
=======================================
  Hits        43973    43973           
  Misses        159      159           

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