Support legacy referrers tag schema fallback

[jonesbusy]

Fix #773

Description

Testing done

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.38%. Comparing base (bdae07d) to head (fe47130).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/ContainerRef.java	60.00%	1 Missing and 1 partial ⚠️
src/main/java/land/oras/Registry.java	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #818      +/-   ##
============================================
+ Coverage     86.99%   87.38%   +0.39%     
- Complexity     1253     1259       +6     
============================================
  Files            50       50              
  Lines          4037     4060      +23     
  Branches        552      556       +4     
============================================
+ Hits           3512     3548      +36     
+ Misses          311      296      -15     
- Partials        214      216       +2     

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