Replace mention of bb8-diesel with diesel_async in README.md

[zekesonxx]

bb8-diesel (https://github.com/overdrivenpotato/bb8-diesel) hasn't been maintained in 3 years and doesn't support modern diesel without going to a random fork. diesel_async is well maintained, has been for several years, and supports bb8 as a backend.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.28%. Comparing base (8639d19) to head (fe455c2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
=======================================
  Coverage   81.28%   81.28%           
=======================================
  Files           6        6           
  Lines         668      668           
=======================================
  Hits          543      543           
  Misses        125      125           

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[djc]

Thanks!