Replace JobArtifact.*_reports scopes with JobArtifact.of_report_type(report_type) scope

What does this MR do and why?

Replaces individual JobArtifact.*_scopes with JobArtifact.of_report_type(report_type) scope. This allows us to parameterize fetching of job artifacts of a given report type.

The MR only changes method names, without any change in the implementation.

Currently, the method takes a symbol representing the report type as a parameter, as listed in Ci::JobArtifacts::REPORT_FILE_TYPES. In a follow up, these symbols should be turned into constants, in order to avoid unnecessary errors.

#371286

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