Update default.mixed to default.qubit.legacy in test_qnode_legacy.py

[andrijapau]

Context:

default.mixed is currently being ported over to the new device API. Therefore, test_qnode_legacy.py should be updated to remove default.mixed and replace it with DefaultQubitLegacy.

Description of the Change:

qml.device("default.mixed", ...) → DefaultQubitLegacy(...)

Benefits: Code clean-up.

Possible Drawbacks: None

[sc-78490]

[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.

[andrijapau]

Do I need a changelog for this? @mudit2812 / @lillian542

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (672ba48) to head (b2927c9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6634   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files         455      455           
  Lines       42820    42820           
=======================================
  Hits        42671    42671           
  Misses        149      149           

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[mudit2812]

One comment, otherwise LGTM

[lillian542]

Looks good to me! I wouldn't think it needs a changelog entry.