Update CDN v5 endpoints to use RFC3339 timestamps

[shamrickus]

This PR updates the cdns v5 TO endpoints to return lastUpdated in the format of RFC3339

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Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Ops

What is the best way to verify this PR?

Query the CRUD CDNs endpoint in API version 5 to confirm RFC3339 format, also query version 4 to confirm the time format has not changed.

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 #7549 (dc10cba) into master (75ec56f) will decrease coverage by 35.23%.
Report is 33 commits behind head on master.
The diff coverage is 11.36%.

@@              Coverage Diff              @@
##             master    #7549       +/-   ##
=============================================
- Coverage     65.05%   29.83%   -35.23%     
  Complexity       98       98               
=============================================
  Files           314      794      +480     
  Lines         12365    84864    +72499     
  Branches        907      905        -2     
=============================================
+ Hits           8044    25318    +17274     
- Misses         3968    57416    +53448     
- Partials        353     2130     +1777     

Flag	Coverage Δ
traffic_ops_integration	69.39% <100.00%> (-0.03%)	⬇️
v3	57.79% <ø> (ø)
v4	79.18% <ø> (ø)
v5	78.52% <100.00%> (-0.12%)	⬇️

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

Files Changed	Coverage Δ
lib/go-tc/cdns.go	0.00% <ø> (ø)
lib/go-tc/federation_resolver.go	45.45% <0.00%> (ø)
traffic_monitor/tmclient/tmclient.go	0.00% <0.00%> (ø)
...raffic_ops/traffic_ops_golang/cdn_lock/cdn_lock.go	0.00% <0.00%> (ø)
...fic_ops/traffic_ops_golang/deliveryservice/keys.go	31.86% <0.00%> (ø)
...affic_ops/traffic_ops_golang/division/divisions.go	8.45% <0.00%> (ø)
...olang/federation_resolvers/federation_resolvers.go	0.00% <0.00%> (ø)
traffic_ops/traffic_ops_golang/login/login.go	10.86% <0.00%> (ø)
traffic_ops/traffic_ops_golang/cdn/cdns.go	28.30% <10.44%> (ø)
...fic_ops/traffic_ops_golang/dbhelpers/db_helpers.go	21.02% <50.00%> (ø)
... and 8 more

... and 473 files with indirect coverage changes

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[jagan-parthiban]

Verified Locally.

[rimashah25]

Code LGTM and comments have been addressed.