Cleanup deprecated termination fields from workspaces agent config gql type

Issues: GraphQL cleanup on rails backend and frontend (#509788 - closed)

What does this MR do and why?

This MR cleans up termination fields (max_hours_before_termination_limit and default_max_hours_before_termination) on the workspaces_agent_config type from Graphql. The individual fields themselves are not marked experimental, but the workspaces_agent_config is marked experimental in the context of ClusterAgent, which is the only entry point to the type.

Client-side cleanup was done in the previous milestone: Remove workspace termination fields from client... (!187507 - merged)

References

Issue: GraphQL cleanup on rails backend and frontend (#509788 - closed)

Previous client side removal: Remove workspace termination fields from client... (!187507 - merged)

Screenshots or screen recordings

How to set up and validate locally

Run bundle exec scripts/remote_development/run-smoke-test-suite.sh, all tests should pass without any errors. In local dev environment, setup remote development workspaces: https://gitlab.com/gitlab-org/workspaces/gitlab-workspaces-docs/-/blob/main/doc/local-development-environment-setup.md

Navigate to http://gdk.test:3000/-/remote_development/workspaces/new and create a workspace Test that workspace is created successfully

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