Update `github.com/go-acme/lego` dependency to v3

[zrhoffman]

This PR resolves #4835 by updating the github.com/go-acme/lego dependency to github.com/go-acme/lego/v3.

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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 TO API test TestAcmeAutoRenew()

PR submission checklist

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