Update `XrayReport` model JSON schema & remove `xray_reports.file_checksum` column

The following discussion from !162313 (merged) should be addressed:

• @mikolaj_wawrzyniak started a discussion: (+2 comments)

  nitpick: Have you considered extracting version string into a const to create SSoT from the start?

  @lma-git: I originally just hardcoded this number in because it will likely be temporary. If we proceed with deprecating Repository X-Ray, we would remove scannerVersion from the payload entirely.

  But extracting it to a const will clean it up and allow me to explain further in a comment. So I'll go ahead and do that.

Considerations

• Do we really need checksums anymore? See #479185 (comment 2140346559).

Proposal

• Update the XrayReport JSON schema:
  • Remove scannerVersion and checksum.
  • Change fileName to file_path.
  • Remove/update the fields above in Ai::Context::Dependencies::ConfigFiles::Base and Ai::RepositoryXray::ScanDependenciesService.
• Remove the xray_reports.file_checksum database column.
  • NOTE: This will likely require a multi-step release approach (docs reference).
  • Related issues:
    • #500462
    • #500465

### Proposal update Due to the database-level constraints caused by the column file_checksum not being nullable, we will encounter issues when we remove checksum from the XRay Report schema as per the proposal above, including failing tests. Therefore, we're adding an additional step to add a migration that allows file_checksum to be nullable before removing the field from the Xray Report schema.