Update validation for escalation rule

What does this MR do?

Adds validation for the elapsed minutes input on escalation rule:

This functionality is behind the feature flag run

rails c
Feature.enable(:escalation_policies_mvc)

The minimal license required is GitLab Premium

To check - navigate to your project=>Operations=>Escalation Policies. Add policy and try to update elapsed seconds input on the scalation rule

Screenshots (strongly suggested)

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

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

Related to #268356 (closed)