create a PL-specific deprecation warning

[timmysilv]

Context:
When we deprecate features, it's hard to identify that we've done it.

Description of the Change:
Replace all deprecation warning categories with the new PennyLaneDeprecationWarning

Benefits:
We can add a pytest.ini filter to make this into failures on plugins!

[pytest]
filterwarnings =
    error::pennylane.PennyLaneDeprecationWarning

(I ran a test locally with a plugin that was setting Observable.return_type and saw that it raised the error)

Possible Drawbacks:
N/A

[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.

[mudit2812]

Thanks! I like this change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2c9a049) 99.64% compared to head (a273577) 99.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4814      +/-   ##
==========================================
- Coverage   99.64%   99.63%   -0.01%     
==========================================
  Files         381      381              
  Lines       34382    34123     -259     
==========================================
- Hits        34260    34000     -260     
- Misses        122      123       +1     

Files	Coverage Δ
pennylane/__init__.py	100.00% <100.00%> (ø)
pennylane/operation.py	97.26% <ø> (-0.02%)	⬇️
pennylane/ops/functions/generator.py	100.00% <ø> (ø)
pennylane/ops/op_math/controlled.py	99.63% <ø> (ø)
pennylane/tape/qscript.py	100.00% <ø> (ø)
pennylane/transforms/__init__.py	100.00% <ø> (ø)
pennylane/transforms/batch_transform.py	96.03% <ø> (ø)
pennylane/transforms/core/transform_dispatcher.py	100.00% <ø> (ø)
pennylane/transforms/op_transforms.py	100.00% <ø> (ø)
pennylane/transforms/qfunc_transforms.py	90.74% <100.00%> (ø)
... and 1 more

... and 40 files with indirect coverage changes

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

[albi3ro]

Worth adding a changelog entry, probably under deprecations, and a comment at the top of deprecations.rst

[albi3ro]

We should potentially have a follow up PR that adds the pytest.ini change to PennyLane itself, but that may take a bunch of test changes.

Hopefully this will make deprecations much cleaner 👍

[trbromley]

How will this change affect the user?

• Will the message they see be different?
• Will they have to update any functionality they have for filtering out warnings?

[albi3ro]

Message is the exact same. Only now the type says PennyLaneDeprecationWarning instead of UserWarning. As it inherits from UserWarning, pre-existing filters will still work. It will just be easier to have finer control over filtering the warnings.