Advance internal API default

[MikeMcQuaid]

• Enables HOMEBREW_USE_INTERNAL_API for users who have run a developer command, increasing coverage before it becomes default.
• Keeps # odeprecated reminders near the env config entry and the defaulting logic.

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[Copilot]

Pull request overview

Updates the environment configuration documentation for HOMEBREW_USE_INTERNAL_API to leave a release-oriented reminder about potentially making the internal API the default in a future Homebrew release.

Changes:

• Adds an odeprecated reminder comment next to HOMEBREW_USE_INTERNAL_API in EnvConfig::ENVS.

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