Migrate from unmaintained gopkg.in/yaml.v3 to go.yaml.in/yaml/v3

[hsinn0]

To replace #131 with conflict resolution.

[hsinn0]

BTW, I did make build test to make sure it is all good.

[peterhaochen47]

AI code review. Seems valid. The "unmaintained gopkg.in/yaml.v3" is still being used.

Issues found
Incomplete migration: credhub/credentials/types_test.go still imports gopkg.in/yaml.v3. This should be updated to go.yaml.in/yaml/v3 to complete the migration.
Dependency status: go.mod still lists both libraries:
go.yaml.in/yaml/v3 v3.0.4 (direct)
gopkg.in/yaml.v3 v3.0.1 (direct)
After migrating the test file, gopkg.in/yaml.v3 can be removed from go.mod if it's no longer used.
...
After this change:
Run go mod tidy again to remove gopkg.in/yaml.v3 if it's no longer needed

[hsinn0]

Removed the remaining use of the package in the new commit.