Change sub-epics to child epics in errors and GraphQL schema

What does this MR do?

In the docs, we refer to sub-epics as "child epics", so change the error messages and API descriptions to match this.

Related issues

Close #235773 (closed)

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

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• 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/gitlab_schema.graphql

@@ -4896,7 +4896,7 @@ Counts of descendent epics.
 """
 type EpicDescendantCount {
   """
-  Number of closed sub-epics
+  Number of closed child epics
   """
   closedEpics: Int
 
@@ -4906,7 +4906,7 @@ type EpicDescendantCount {
   closedIssues: Int
 
   """
-  Number of opened sub-epics
+  Number of opened child epics
   """
   openedEpics: Int