Fix `project` attribute of `StoreScanService` class

What does this MR do?

While building this feature, I got feedback to extend the service class from ::Base service class which has an attr_accessor defined for the project attribute therefore even though we didn't have a project set for this class, it wasn't raising an error and we couldn't catch the issue, neither with unit tests nor with manual tests on staging because on the test and staging environments the feature flag was globally enabled.

Related to #267021 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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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

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• [-] 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
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