Move masked variable regex warning to help text

What does this MR do and why?

For #364856 (closed)

Move masked variable regex warning to help text

In the variable settings modal,
instead of making the warning bold when masked is checked
show help text next to the variable value input

Changelog: changed

Screenshots or screen recordings

Before	After
Screen_Recording_2023-06-01_at_12.58.16_PM	Screen_Recording_2023-06-01_at_12.52.37_PM

How to set up and validate locally

1. navigate to a project, and in the sidebar choose Settings => CI/CD
2. expand the variables settings section
3. click the Add variable button to open the variable modal
4. toggle the Mask variable ("Mask this variable in job logs if it meets regular expression requirements.") checkbox under different conditions:
   • the help text "Value must meet regular expression requirements to be masked." should appear below the variable value field whenever the box is checked
   • the validation message "This variable value does not meet the masking requirements." should appear below the variable value field (but above the help text) if the value is not blank and does not match the regex

MR acceptance checklist

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#418331 (closed)