Deprecate event_loop_policy fixture

[tjkuson]

Emit a deprecation warning when a user uses the event_loop_policy fixture. Add deprecation notice to documentation.

One documentation code snippet tests was failing due to the change. As I was about to update it, I noticed that the snippet wasn't actually used anywhere, so I deleted it.

Closes #1419

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.56%. Comparing base (4919983) to head (651f65c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1421      +/-   ##
==========================================
+ Coverage   94.53%   94.56%   +0.03%     
==========================================
  Files           2        2              
  Lines         512      515       +3     
  Branches       61       62       +1     
==========================================
+ Hits          484      487       +3     
  Misses         22       22              
  Partials        6        6              

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