Clean up uniqueness validations for service type

What does this MR do?

• Remove on: :create for project services, this can now always be validated after we've removed all duplicates.

  • #290008 (closed)

• Unify the validation syntax for all service types.

  • The queries are the same as before for project_id / group_id.
  • For template / instance the order of the WHERE conditions is swapped, but they can still use the same indexes as before.

Related to #207385 (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