Add issuableTitleUpdated subscription to GraphQL

What does this MR do and why?

Adds an issuableTitleUpdated subscription. Only triggered when a work item title is updated now.

How to set up and validate locally

No easy way to validate locally as currently there's no way to test subscriptions via GraphiQL

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 #343668 (closed)