Resolve "Hide variables in UI by default"

What does this MR do?

The goal of this MR is to:

1. Hide trigger variable values by default in the UI.
2. Provide an option to reveal the values if the user is a Maintainer.
3. Remove the variable values from the request payload if the user is not a Maintainer.

What are the relevant issue numbers? 20422

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #20422 (closed)