prometheus-operator: Get rid of `github.com/pkg/errors` dependency

https://github.com/pkg/errors has been archived and is no longer maintained.

At least 2 options are available:

• ~~Use a drop-in replacement like https://github.com/emperror/errors.~~

• Use the errors package from the standard Go library.
• Add a depguard rule in the golang-cilint configuration to prevent future use of github.com/pkg/errors.

See also https://github.com/thanos-io/thanos/issues/5176