Validate rotation length for on call scheduling

What does this MR do?

Adds frontend validation for rotation length. Converts the length type from string to number to avoid some parsing weirdness.

This feature is behind a feature flag: :oncall_schedules_mvc.

To test:

• Enable the feature flag
• Navigate to <project-slug>/-/oncall_schedules
• Create a new schedule if you don't already have one
• Create a new rotation. Add a non positive integer into the length input field.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/322137

Screenshots (strongly suggested)

before	after

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] 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
• [n/a] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team