[Backend] Replace .ContainsKey with .TryGetValue/.TryAdd

[imnasnainaec]

https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1854
In preparation for updated pragma suggestions with .NET 8.0 in #3046.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 44.82759% with 16 lines in your changes missing coverage. Please review.

Project coverage is 75.02%. Comparing base (c6f2191) to head (119e4ff).

Files	Patch %	Lines
Backend/Services/StatisticsService.cs	25.00%	10 Missing and 2 partials ⚠️
Backend/Services/LiftService.cs	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3052      +/-   ##
==========================================
+ Coverage   74.99%   75.02%   +0.02%     
==========================================
  Files         271      271              
  Lines       10415    10394      -21     
  Branches     1235     1230       -5     
==========================================
- Hits         7811     7798      -13     
+ Misses       2242     2237       -5     
+ Partials      362      359       -3     

Flag	Coverage Δ
backend	84.04% <44.82%> (+0.09%)	⬆️
frontend	66.83% <ø> (ø)

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[jasonleenaylor]

Reviewed 1 of 5 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)