Fix SAST report format version

What does this MR do?

Fix SAST report format version stored as metadata_version in the vulnerability_occurrences record. This value is aligned with what is planned for report versioning.

There is not stored data yet so there is no harm to change that value without DB migration.

What are the relevant issue numbers?

#6717 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?