Rollback MongoDB sharding rules for users collection

[sejongk]

What this PR does / why we need it:
This PR rolls back the MongoDB sharding rules for the Users collection to prevent unnecessary sharding of the collection. Similar to the Projects collection, the Users collection is expected to store a relatively small amount of data.

Which issue(s) this PR fixes:

Addresses #673

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (6651d42) 50.72% compared to head (71c83c3) 50.73%.

Files	Patch %	Lines
server/backend/database/memory/database.go	68.42%	4 Missing and 2 partials ⚠️
server/backend/database/mongo/client.go	66.66%	4 Missing and 1 partial ⚠️
server/rpc/admin_server.go	70.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #770   +/-   ##
=======================================
  Coverage   50.72%   50.73%           
=======================================
  Files          70       70           
  Lines       10210    10234   +24     
=======================================
+ Hits         5179     5192   +13     
- Misses       4509     4517    +8     
- Partials      522      525    +3     

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[hackerwins]

Thanks for your contribution.