Allow to update issuable severity via GraphQL

What does this MR do?

Implements #238564 (closed)
Contributes to #229402 (closed)

Allows to update issuable severity via GraphQL

Decisions made during the implementation:

1. At first, I was thinking to amend an existing IssueUpdate mutation because that's the power of GraphQL to have a single mutation for many fields and you just pick the fields you need to update. But it seems to me we tend to split some mutations: SetAssignee, SetConfidential, SetDueDate, SetLocked, SetSubscription. Since Severity is going to be used as a widget in the right sidebar, it seems to me it fell well into that category.

2. Even though for now we support updating severity only for the incident, the mutation still named 'IssueSetSeverity'. It is based on the discussion, the suggestion to design a severity widget with the ability to support all other issue types. Although, we have separate mutations for MergeRequest.

Screenshots

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

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:

• ~~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