Move common request specs into shared_examples

What does this MR do?

The request specs for the API to delete the self-monitoring project will be similar to the specs for the API to create the self-monitoring project.

In preparation for the MR (!21431 (merged)) that adds the API to delete the self-monitoring project, move the request specs for the create API into shared_examples so that they can be reused.

Issue: #32351 (closed)

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

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