update referrer manifest descriptor size

[MinerYang]

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

#20187

curl -k -u 'admin:pwd' https://xx.xxx.xxx.xx/v2/library/hello-world/referrers/sha256:d37ada95d47ad12224c205a938129df7a3e52345828b4fa27b03a98825d1e2e7 | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   300  100   300    0     0  26073      0 --:--:-- --:--:-- --:--:-- 27272
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:8a12406f6f62e297a93400610fa6a136c7fcf6a0956219bc929676014d2f2fc3",
      "size": 1620,
      "artifactType": "application/vnd.oci.image.config.v1+json"
    }
  ]
}

/data/registry/docker/registry/v2/blobs/sha256$ wc -c <  8a/8a12406f6f62e297a93400610fa6a136c7fcf6a0956219bc929676014d2f2fc3/data
1620

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.21%. Comparing base (b7b8847) to head (4889c28).
Report is 120 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #20207       +/-   ##
===========================================
- Coverage   67.56%   46.21%   -21.36%     
===========================================
  Files         991      246      -745     
  Lines      109181    13565    -95616     
  Branches     2719     2781       +62     
===========================================
- Hits        73768     6269    -67499     
+ Misses      31449     6980    -24469     
+ Partials     3964      316     -3648     

Flag	Coverage Δ
unittests	46.21% <ø> (-21.36%)	⬇️

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see 1237 files with indirect coverage changes

[wy65701436]

lgtm