Improve validation messages for CI variables and fix failing spec

What does this MR do and why?

This will add more specific validation messages to the Value field in the CI Variable form.

This MR has two commits:

• ccd14ba1d513611c3f2e43bb7eb279e25e17c707: Re-introduces the change from !135823 (merged), which was reverted due to a failing spec. There are no code changes to this since then.
• a9f87f948c686cb2b259cc4e647a4a1b54bd2ffd: Fixes the failing spec related to this change.

Addresses #431837 (closed).

No changelog since this is fixing a change introduced in the same milestone.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to Settings > CI/CD > Variables and click on the Add variable button.
2. Make sure Mask variable is checked.
3. Enter something in the Value field. Verify that the validation messages shown adhere to our masking requirements.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.