Use mongo driver 4 rc

[aljones15]

Awaits:

• MAJOR: Upgrade to node mongodb driver 4 bedrock-mongodb#92

Todo:

• Test with auth turned on in mongo server 6.

[codecov-commenter]

Codecov Report

Merging #42 (bec9f4d) into main (8c66230) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   93.37%   93.36%   -0.01%     
==========================================
  Files           8        8              
  Lines         800      799       -1     
==========================================
- Hits          747      746       -1     
  Misses         53       53              

Impacted Files	Coverage Δ
lib/challenges.js	87.24% <100.00%> (-0.07%)	⬇️

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