Updated gopkg.in/yaml.v2 module to 2.2.8 and updated go version to 1.15.13

[rimashah25]

What does this PR (Pull Request) do?

• This PR is not related to any Issue
• This PR makes the Go components of Traffic Control build using Go version 1.15.13 and updates the gopkg.in/yaml.v2 dependencies.. due to a security issue: Upgrade gopkg.in/yaml.v2 to 2.2.8+ 99designs/gqlgen#1485

Which Traffic Control components are affected by this PR?

• CDN in a Box
• Grove
• Traffic Control Client (Go)
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Stats
• CI tests for Go components

What is the best way to verify this PR?

Run unit tests and API tests. Since this is only a patch-level version update, the only changes were bugfixes. Breaking changes would be unexpected.

If this is a bug fix, what versions of Traffic Control are affected?

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[zrhoffman]

Checks all pass except TestDeliveryServices in the API tests, which is fixed in #5957. Merging now because the go.sum changes frequently.