include a copyright holder in the boilerplate; remove year

see:
https://github.com/etcd-io/etcd-operator/pull/456#discussion_r3776398411

i propose all license headers to be changed to:
```
Copyright The etcd-operator Authors
```

this aligns with k8s.

alternatively align it with etcd, but there must be a copyright holder in the title or this is pointless.

https://github.com/etcd-io/etcd-operator/blob/main/hack/boilerplate.go.txt

additionally best to add a `./hack/verify-boilerplate.sh` and make it part of CI.

Start with hack/boilerplate.go.txt and inspect the existing license headers and CI configuration. Update the headers to include the proposed copyright holder and remove the year, add hack/verify-boilerplate.sh, and confirm the verification runs successfully in CI.

Written by the indexing model from the issue text.