Automatic merge from submit-queue (batch tested with PRs 38154, 38502)

Rename "release_1_5" clientset to just "clientset"

We used to keep multiple releases in the main repo. Now that [client-go](https://github.com/kubernetes/client-go) does the versioning, there is no need to keep releases in the main repo. This PR renames the "release_1_5" clientset to just "clientset", clientset development will be done in this directory.

@kubernetes/sig-api-machinery @deads2k 

```release-note
The main repository does not keep multiple releases of clientsets anymore. Please find previous releases at https://github.com/kubernetes/client-go
```