dep: bump gopkg.in/yaml to v3

[maxmoehl]

• [x] Read the Contributing document.

Summary

Bump from yaml.v2 to yaml.v3

See also:

• cloudfoundry/cf-tcp-router#32
• cloudfoundry/gorouter#436
• cloudfoundry/route-registrar#56
• cloudfoundry/routing-api#64

Backward Compatibility

Breaking Change? No

[maxmoehl]

I've ran the tests to ensure the dependant packages don't break.

[peanball]

Merging once the other packages have been bumped, as this removes the yaml.v2 packages. V3 is also available already in routing-release, so the bumps of the submodules will work fine.