Rename stage field in the configuration to deploymentId

[amarthadan]

We're using the stage field to distinguish between deployments anyway so we should make it official. The change is pretty much just the rename of the field but we should mention in the documentation that it is used to distinguish between the deployments and should be unique for each Airnode/deployment.

Slack discussion

[bbenligiray]

This should be blocked by the cloud resource naming rework (because the assumption for this issue was that stage needs to be temporally unique, which may not be the case)