Mark TokenAcquirer credentials as obsolete and add v4 migration guide (#3534)

* Initial plan

* Initial plan for v4 obsolete TokenAcquirer credentials

Co-authored-by: jmprieur <13203188+jmprieur@users.noreply.github.com>

* Add Obsolete attributes to TokenAcquirer credentials and update documentation

Co-authored-by: jmprieur <13203188+jmprieur@users.noreply.github.com>

* docs(v4): unify migration guide with codebase audit & analyzer preview

* Update MIGRATION_GUIDE_V4.md

Co-authored-by: Keegan <Keegan.Caruso@microsoft.com>

* Update MIGRATION_GUIDE_V4.md

Co-authored-by: Keegan <Keegan.Caruso@microsoft.com>

* Apply suggestion from @jmprieur

* Update migration guide by removing pre-commit hook

Removed optional pre-commit hook section and updated future analyzer integration section.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jmprieur <13203188+jmprieur@users.noreply.github.com>
Co-authored-by: Jean-Marc Prieur <jmprieur@microsoft.com>
Co-authored-by: Keegan <Keegan.Caruso@microsoft.com>

Author: 3 people