Update Go version to 1.21.0

[zrhoffman]

This PR makes the Go components of Traffic Control build using Go version 1.21 (no golang.org/x/ dependency updates needed)

See the Go 1.21 release notes and milestone:

The latest Go release, version 1.21, arrives six months after Go 1.20. Most of its changes are in the implementation of the toolchain, runtime, and libraries. As always, the release maintains the Go 1 promise of compatibility; in fact, Go 1.21 improves upon that promise. We expect almost all Go programs to continue to compile and run as before.

Which Traffic Control components are affected by this PR?

• Traffic Control Cache Config (t3c, formerly ORT)
• Traffic Control Health Client (tc-health-client)
• Traffic Control Client
• Traffic Monitor
• Traffic Ops
• Traffic Stats
• Grove
• CDN in a Box - Enroller
• CI tests for Go components
• Build system - Go version in builder images

What is the best way to verify this PR?

Run unit tests and API tests. Since Because of the Go 1 promise of compatibility, breaking changes would be unexpected.

If this is a bugfix, which Traffic Control versions contained the bug?

PR submission checklist

• Existing tests are sufficient, no additional tests necessary
• The documentation only mentions the major Go version, no documentation updates necessary.
• The changelog already mentions updating to Go 1.20, no additional changelog message necessary.
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Merging #7705 (43ce1ad) into master (75ec56f) will decrease coverage by 35.50%.
Report is 52 commits behind head on master.
The diff coverage is 6.11%.

@@              Coverage Diff              @@
##             master    #7705       +/-   ##
=============================================
- Coverage     65.05%   29.56%   -35.50%     
  Complexity       98       98               
=============================================
  Files           314      589      +275     
  Lines         12365    73374    +61009     
  Branches        907       90      -817     
=============================================
+ Hits           8044    21691    +13647     
- Misses         3968    49610    +45642     
- Partials        353     2073     +1720     

Flag	Coverage Δ
golib_unit	52.75% <0.00%> (?)
grove_unit	12.02% <ø> (?)
t3c_unit	6.04% <6.32%> (?)
traffic_monitor_unit	26.22% <ø> (?)
traffic_ops_integration	69.39% <91.17%> (-0.03%)	⬇️
traffic_ops_unit	22.84% <4.27%> (?)
traffic_portal_v2	?
traffic_stats_unit	10.76% <ø> (?)
unit_tests	26.27% <4.19%> (-47.51%)	⬇️
v3	57.79% <ø> (ø)
v4	79.18% <ø> (ø)
v5	78.52% <91.17%> (-0.12%)	⬇️

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

Files Changed	Coverage Δ
cache-config/t3c-apply/config/config.go	0.00% <0.00%> (ø)
lib/go-atscfg/parentdotconfig.go	65.76% <ø> (ø)
lib/go-tc/cdns.go	0.00% <ø> (ø)
lib/go-tc/deliveryservice_servers.go	0.00% <0.00%> (ø)
lib/go-tc/federation_resolver.go	45.45% <0.00%> (ø)
lib/go-tc/log.go	0.00% <0.00%> (ø)
...c_ops/traffic_ops_golang/cachegroup/cachegroups.go	14.93% <0.00%> (ø)
...raffic_ops/traffic_ops_golang/cdn_lock/cdn_lock.go	0.00% <0.00%> (ø)
...ops/traffic_ops_golang/deliveryservice/eligible.go	55.27% <0.00%> (ø)
...fic_ops/traffic_ops_golang/deliveryservice/keys.go	31.86% <0.00%> (ø)
... and 21 more

... and 471 files with indirect coverage changes

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[shamrickus]

LGTM

• Relevant go components use 1.21
• Builds are successful