Remove dependency on gopkg.in/yaml.v3

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See chore(deps): do not use the experimental package (!2614 - merged):

I was about to add a commit to change gopkg.in/yaml.v3 to go.yaml.in/yaml/v3 but then noticed it is only used in a single place... and that is just some example code (config.NewExtension used in config/examples/basic_usage.go)! It's a bit too much to import such a heavy library just for an example if you ask me. I think it'd be worth moving the example into just docs or a separate Go module.