Replace stub with bootstrapFiles for Eloquent class alias

[firecow]

Summary

• Replaces stubFiles with bootstrapFiles for the Eloquent class alias
• Stubs only override PHPDocs of existing classes, they don't make classes discoverable
• bootstrapFiles runs class_alias() which properly registers Eloquent so phpstan can find it
• Fixes flaky class.notFound for @mixin \Eloquent in CI

Test plan

• Verify phpstan no longer reports class.notFound for @mixin \Eloquent in CI