DAST on-demand scans site profile form - refactor form validation

What does this MR do?

• Adds a new vue-directive that allows to leverage the browser's native constraints-validation API
• Switches the manual form-validation to use the new directive within the DAST on-demand scans site-profile form
• Removes the disabling of the submit button (see #259684 (closed))

Screen Recording

validation

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #259684 (closed)