Serialize datetimes with Z suffix

[tw4l]

Use timezone aware datetimes instead of timezone naive datetimes:

• Update mongodb client to use tz-aware conversion
• Convert dt_now() to return timezone aware UTC date
• Rename to_k8s_date -> date_to_str, just returns ISO UTC date with 'Z' (instead of '+00:00' suffix)
• Rename from_k8s_date -> str_to_date, returns timezone aware date from str
• Standardize all string<->date conversion to use either date_to_str or str_to_date
• Update frontend to assume iso date, not append 'Z' directly
• Update tests to check for 'Z' suffix on some dates

[tw4l]

Pushed a few frontend fixes for areas where dates were still having Z manually appended that I missed on the first pass. Otherwise it seems good! I spun up a local instance off of main, created some workflows, collections, uploads, etc., and then redeployed with this branch, and all seems good.