chore: add workflow to deprecate v6 on npm

[tusharpandey13]

Summary

• Adds a workflow_dispatch GitHub Actions workflow that runs npm deprecate to mark all auth0@>=6.0.0 <7.0.0 versions as deprecated on the npm registry.
• One-time operation: trigger manually from the Actions tab after merging, no automation or scheduled runs.

Pre-merge checklist

• NPM_TOKEN secret added to repo (classic automation token, scoped to auth0 package)
• v7_MIGRATION_GUIDE.md exists on master (URL in deprecation message is valid)

How to run

1. Merge this PR into master.
2. Go to Actions → Deprecate v6 on npm → Run workflow (select master).
3. Verify: npm view auth0@6.4.0 deprecated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (e1e90b8) to head (94c24e7).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1407    +/-   ##
========================================
  Coverage   89.68%   89.68%            
========================================
  Files         439      439            
  Lines       21080    21080            
  Branches    10327     9990   -337     
========================================
  Hits        18905    18905            
  Misses       2175     2175            

Flag	Coverage Δ
alltests	89.68% <ø> (ø)

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