Make sure keys from the existing pipeline are stringified

What does this MR do and why?

Currently, in Cloud Seed, after configuring the pipeline through the "Enable Cloud Run deployments" button, the generated pipeline contains keys with : prefix (see below).

This MR fixes that through Hash.deep_stringify_keys before the yaml dump.

Example commit: astao/rails-playground!1 (4486de94)

Before:

After:

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