Use RFC 3339 for lastUpdated timestamp in /statuses

[zrhoffman]

This PR resolves #7706 by using the timestamp format specified in RFC 3339 for the lastUpdated field of the /api/5.0/statuses endpoint.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Run the statuses API tests

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Merging #7707 (5e30a16) into master (849d166) will decrease coverage by 0.93%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #7707      +/-   ##
============================================
- Coverage     29.78%   28.85%   -0.93%     
  Complexity       98       98              
============================================
  Files           802      745      -57     
  Lines         85789    84214    -1575     
  Branches        952      952              
============================================
- Hits          25550    24302    -1248     
+ Misses        58099    57852     -247     
+ Partials       2140     2060      -80     

Flag	Coverage Δ
traffic_ops_integration	65.66% <100.00%> (-3.74%)	⬇️
v3	57.79% <ø> (ø)
v4	?
v5	78.52% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
lib/go-tc/statuses.go	0.00% <ø> (ø)
traffic_ops/traffic_ops_golang/routing/routes.go	95.52% <ø> (-0.02%)	⬇️
traffic_ops/traffic_ops_golang/status/statuses.go	24.75% <ø> (ø)
traffic_ops/v5-client/status.go	100.00% <100.00%> (ø)

... and 57 files with indirect coverage changes

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[shamrickus]

LGTM
Code looks good and the v5 endpoint is responding properly with the new format.