Auto DevOps should use `REPLICAS`

Description

Now that we have environment-specific secret variables, users can set REPLICAS with different values for different environments and we should need to use PRODUCTION_REPLICAS, etc. Auto DevOps should look for REPLICAS. To maintain backwards compatibility, it should look for both, with precedence going to the more specific one, PRODUCTION_REPLICAS.

Proposal

Links / references

https://docs.gitlab.com/ee/topics/autodevops/#advanced-replica-variables-setup