fix: Convert Devise.sign_out_via to String

[tvdeyen]

Since lastest Alchemy main (8.1.0.a) this needs
to be a String and cannot be a Symbol.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (fa27d9e) to head (566cd0f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage   98.62%   98.62%           
=======================================
  Files          11       11           
  Lines         291      291           
=======================================
  Hits          287      287           
  Misses          4        4           

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