Remove placeholder in Container Registry cleanup policy text field

What does this MR do and why?

This MR:

• Remove the placeholder in the name regex field
• Move the errors from the snackbar to under the form (when the errors are not classified by the API we assume they are for nameRegex

Screenshots or screen recordings

Kapture_2022-03-08_at_17.43.43

How to set up and validate locally

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

1. Go to a project
2. Go to settings => packages and registries
3. Use the cleanup policy settings

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.

Related to #336663 (closed)