Update Go version to 1.15.7

[zrhoffman]

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 1.15.7.

See the Go 1.15.7 release notes:

• cmd/go: packages using cgo can cause arbitrary code execution at build time
• crypto/elliptic: incorrect operations on the P-224 curve

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

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.

The following criteria are ALL met by this PR

• Existing tests are sufficient, no additional tests necessary
• The documentation only mentions the Go version with minor version precision, no documentation updates necessary.
• The changelog already mentions updating to Go 1.15, no additional changelog message necessary.
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[shamrickus]

LGTM