Change usage of CI/CD variables terminology in package docs

What does this MR do?

In the linked issue, we are trying to standardize how we talk about CI/CD variables. We use a variety of terminology, but the most common issue is the use of "Environment variable", which can be confused with the Environments feature, or variables in other environments than CI/CD.

This MR standardizes the usage of variables in the package docs, following this convention:

• The first time CI/CD variables are mentioned, use CI/CD variable
• Any further mentions can use simply variable, as long as the context continues to be clear that it's CI/CD variables.
• If Environment variable is used to describe variables within an environment, like in a Kubernetes Pod or a container, then it is left unchanged.

Related issues

Related to #321011 (closed)

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