Fix broken label uniqueness label migration

The previous implementation of the migration failed on staging because
the migration was attempted to remove labels from projects that did not
actually have duplicates. This occurred because the SQL query did not
account for the project ID when selecting the labels.

To replicate the problem:

1. Disable the uniqueness validation in app/models/label.rb.
2. Create a duplicate label "bug" in project A.
3. Create the same label in project B with label "bug".

The migration will attempt to remove the label in B even if there are no duplicates.

To fix the issue, include the project ID when selecting the labels.

Closes #23609

See merge request !7030