Change usage of CI/CD variables terminology in Secure 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 secure 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.

This means I had to rename one file:

• doc/security/cicd_environment_variables.md -> doc/security/cicd_variables.md

As a side effect, since I was editing some tables that contained nothing but variables, I also fixed the alignment in them to make them easier to maintain.

Related issues

Related to #321011 (closed)

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