Clean up GraphQL access error specs with shared example

What does this MR do?

uses the shared example introduced in !39408 (merged) to avoid us having to and change a lot of different strings if our error message ever changes.

in the interests of incremental improvement, i've left out several specs (see below) that require more reworking to work with the new shared example.

ee/spec/requests/api/graphql/mutations/issues/set_weight_spec.rb
ee/spec/requests/api/graphql/mutations/issues/set_epic_spec.rb
spec/requests/api/graphql/mutations/boards/issues/issue_move_list_spec.rb
spec/requests/api/graphql/mutations/issues/set_confidential_spec.rb
spec/requests/api/graphql/mutations/issues/set_due_date_spec.rb

Related Issue(s)

#238959 (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

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