docs: add v6 migration guide for breaking changes in v6.0.0

[ankita10119]

Changes

Adds a v6_MIGRATION_GUIDE.md to help consumers migrate from 5.x to 6.x. This is needed as v6.0.0 introduces breaking changes that were previously shipped in minor releases under v5.x.

Breaking changes documented:

1. ConnectionAttributeIdentifier removed - replaced with three identifier-specific types (EmailAttributeIdentifier, PhoneAttributeIdentifier, UsernameAttributeIdentifier) to correctly type the default_method field per identifier. Before/after code examples included.
2. PhoneProviderProtectionBackoffStrategyEnum.None: "none" → Default: "default" - reflects an API-side change confirmed in the api2 spec. Before/after code examples included.

References

• PR introducing the breaking changes: feat: add org role members, third-party client access, passwordless type fixes, and OIDC connection updates #1363

Testing

Documentation-only change, no code changes.

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.72%. Comparing base (279fd80) to head (c295b84).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1368    +/-   ##
========================================
  Coverage   89.72%   89.72%            
========================================
  Files         433      433            
  Lines       20115    20115            
  Branches     9426     9786   +360     
========================================
  Hits        18048    18048            
  Misses       2067     2067            

Flag	Coverage Δ
alltests	89.72% <ø> (ø)

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[developerkunal]

LGTM