change Referrers Content-Type to application/vnd.oci.image.index.v1+json

[MinerYang]

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

Fixes

• Conformance test issue -- GET request to nonexistent blob should result in empty 200 response #19176
• Conformance test issue -- GET request to existing blob should yield 200 #19177
• Conformance test issue -- GET request to existing blob with filter should yield 200 #19178

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

Merging #19212 (f329806) into main (dd90313) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #19212      +/-   ##
==========================================
- Coverage   67.39%   67.38%   -0.02%     
==========================================
  Files         993      993              
  Lines      108896   108896              
  Branches     2751     2751              
==========================================
- Hits        73394    73382      -12     
- Misses      31555    31568      +13     
+ Partials     3947     3946       -1     

Flag	Coverage Δ
unittests	67.38% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/server/registry/referrers.go	68.14% <100.00%> (ø)

... and 9 files with indirect coverage changes

[wy65701436]

lgtm

[chlins]

lgtm