Reduce Pod and Deployment cached data

Summary

As part of an investigation to find the biggest ReactiveCaching offenders, from a RDB dump, we found out that 2% of our environment cached keys go way above the limit of 1 MB that we plan to impose for ReactiveCaching on https://gitlab.com/gitlab-org/gitlab/issues/14015.

Proposed Solution

Currently we cache all the information that comes from pods. We probably don't need all of it and we could save only what's really needed.

https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/environment.rb#L197

https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/clusters/platforms/kubernetes.rb#L95

Risks

We need to test well the use of terminals after the refactoring. It might be a good idea to check if the specs for the terminals are good enough.