chore: remove registration extension form setting

[BryanttV]

Description

This PR removes the deprecated REGISTRATION_EXTENSION_FORM setting and consolidates profile extension functionality under the PROFILE_EXTENSION_FORM setting. This change is part of the Drop REGISTRATION_EXTENSION_FORM Deprecation ticket.

Changes

Core Changes

• Removed REGISTRATION_EXTENSION_FORM setting: Deprecated setting has been removed from the codebase.
• Consolidated to PROFILE_EXTENSION_FORM: All profile extension functionality now uses a single, consistent setting.
• Renamed functions for clarity: Updated function names from "registration extension" to "profile extension" terminology to better reflect their purpose.

Test Updates

• Fixed test patching: Updated test decorators to patch get_profile_extension_model instead of the non-existent get_extended_profile_model
• Renamed test references: Updated all test references from "registration extension form" to "profile extension form"
• Improved test coverage: Enhanced tests for extended profile handling and validation

Breaking Changes

⚠️ Important: Sites using REGISTRATION_EXTENSION_FORM must migrate to PROFILE_EXTENSION_FORM

Supporting information

• [DEPR]: Drop REGISTRATION_EXTENSION_FORM django setting #38409

Testing instructions

You can follow the instructions in this PR

Deadline

None

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