Actions must have an expiration time or they will not be dispatched to agents

### Summary
[It was discovered](https://github.com/elastic/kibana/issues/170322#issuecomment-1810483659) that an UPGRADE action with no expiration doesn't get dispatched to elastic agent, as fleet-server dispatches only actions where `expiration > now`. [See logic here](https://github.com/elastic/fleet-server/blob/a465d093a882d8b4a9ec1ae97d8494b0d2f9ae39/internal/pkg/dl/actions.go#L117)

So we should allow actions without expiration so in the future it will be possible to completely remove the expiration from Fleet.