Update iteration cadence mutation [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds GraphQL support for updating iteration cadences. #322742 (closed)

Screenshots (strongly suggested)

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

doc/api/graphql/reference/index.md

@@ -4430,6 +4430,16 @@ Autogenerated return type of UpdateIssue.
 | `errors` | String! => Array | Errors encountered during execution of the mutation. |
 | `issue` | Issue | The issue after mutation. |
 
+### UpdateIterationCadencePayload
+
+Autogenerated return type of UpdateIterationCadence.
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `clientMutationId` | String | A unique identifier for the client performing the mutation. |
+| `errors` | String! => Array | Errors encountered during execution of the mutation. |
+| `iterationCadence` | IterationsCadence | The updated iterations cadence. |
+
 ### UpdateIterationPayload
 
 Autogenerated return type of UpdateIteration.