Enable CA1854 analyser

[martincostello]

I spotted a new analyser being dogfooded in dotnet/runtime#70157 that suggests using TryGetValue() on dictionaries over ContainsKey() & the indexer.

I've spotted a few uses of the latter pattern in the repo from time to time, so it would be good to enable that analyser and fix-up any warnings from it once the relevant change has been ingested into this repo.

[mkArtakMSFT]

Thanks for bringing this up, @martincostello.
@dotnet/aspnet-api-review crew, any objections to enabling this analyzer on the repo?
/cc @adityamandaleeka, @rafikiassumani-msft, @captainsafia, @JamesNK, @javiercn

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[BrennanConroy]

This was done in #44799