Remove linuxbrew automatic git repo migration

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

---

The JSON API provides an easy alternative for any users who still haven't migrated.

---

This removes remaining Linuxbrew migration handling (other than diagnostic message) following:

• Remove migration of linuxbrew-core formulae #21195

[Copilot]

Pull request overview

This PR removes the automatic git repository migration from linuxbrew-core to homebrew-core. With the JSON API now providing an easy alternative for users who haven't migrated, the automatic migration code is no longer necessary. Users still using linuxbrew-core should either use the JSON API (by ensuring HOMEBREW_NO_INSTALL_FROM_API is not set) or manually update their repository remote.

Key changes:

• Removed automatic migration detection and execution logic
• Updated diagnostic message to guide users toward manual migration or JSON API usage
• Cleaned up environment variable documentation

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/extend/os/linux/diagnostic.rb	Updated error message to direct users to either unset HOMEBREW_NO_INSTALL_FROM_API or manually update the repository remote
Library/Homebrew/cmd/update.sh	Removed migration logic that was triggered by HOMEBREW_LINUXBREW_CORE_MIGRATION and updated environment variable documentation
Library/Homebrew/cmd/update-report.rb	Removed code that automatically re-ran brew update for linuxbrew-core migration
Library/Homebrew/brew.sh	Removed detection code that set HOMEBREW_LINUXBREW_CORE_MIGRATION when a linuxbrew-core repository was detected

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MikeMcQuaid]

Thanks again!