Deprecating Legacy Devices

[Shiro-Raven]

Overview of deprecation changes:

• Added general fixture to conftest.py for capturing warnings. This should capture any uncaptured warnings from the tests (read, the deprecation warnings from the legacy devices), and therefore works as expected and shouldn't cause any issues.
• Restructured interface tests: moved legacy tests to a separate folder, pulled out tests in the default.qubit folder one level, and added missing tests from the legacy tests to the DQ's tests. This should make the removal process much easier by simply deleting a folder and pulling a bunch of files up a directory.
• Updated some examples from the pulse module (and related misc. files) that were relying on legacy devices and updated the code block accordingly.
• Switched from default.qubit.legacy to default.mixed for tests testing the old device interface, and not simply the legacy itself.

[sc-66694]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (a456fce) to head (fa8d10a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5997      +/-   ##
==========================================
- Coverage   99.65%   99.65%   -0.01%     
==========================================
  Files         428      428              
  Lines       41160    40883     -277     
==========================================
- Hits        41020    40742     -278     
- Misses        140      141       +1     

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[mudit2812]

Looks good. Minor non-blocking comment above.

[albi3ro]

This makes me so happy :)