Use "Environment spec" everywhere instead of "Environment scope"

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Problem

Today, there is a naming mismatch on the functionality for scoping environments.

• Clusters calls "Environment scope"
• Secret Variables calls "Environment scope"
• Feature Flag calls "Environment spec"

We should use the same terminology in order to avoid confusion.

NOTE: We use the word at the UI-level, not just Documentaion-level. So that we need to rename frontend components as well