fix: require auth0/version so client construction works and correct rest-client changelog note

[kishore7snehil]

Changes

Two fixes reported against v6.

• lib/auth0/mixins/headers.rb now requires auth0/version before referencing Auth0::VERSION. The telemetry header is built during every client construction (Initializer#initialize -> client_headers -> telemetry), and the gem entrypoint never loaded the version file, so a plain require "auth0" followed by Auth0::Client.new(...) raised NameError: uninitialized constant Auth0::VERSION. The require lives in headers.rb (a .fernignore-protected file) so it survives Fern regeneration.
• Corrected the v6 changelog note that claimed the rest-client dependency was removed. Only the Management API client moved to net/http. The Authentication API still uses rest-client, and it remains a declared dependency in the gemspec.

References

• Closes v6.0.0: Auth0::Client.new raises NameError: uninitialized constant Auth0::VERSION (entrypoint never requires auth0/version) #786
• Closes v6 still depends on RestClient contrary to the changelog info #785

Testing

• Reproduced v6.0.0: Auth0::Client.new raises NameError: uninitialized constant Auth0::VERSION (entrypoint never requires auth0/version) #786 in a clean process (require "auth0" with no require "auth0/version"), confirmed the NameError, applied the fix, and confirmed construction succeeds with Auth0::VERSION resolving to 6.1.0.
• Verified end to end against a live tenant: constructed the client, obtained an M2M token, and made a Management API call, all with only require "auth0".
• Ran the full WireMock wire suite locally (445 runs, 0 failures).

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of Ruby

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
• Rubocop passes on all added/modified files
• All active GitHub checks have passed