Drop project#build_coverage_regex column

Problem

Following up #17633 (closed) where we deprecated our project setting for CI coverage detection.

This issue is about cleaning up our codebase as explained in our documentation.

Solution

• Make the column not accessible anymore: ignore_columns :build_coverage_regex, remove_after: '2022-07-22', remove_with: '15.0' in project.rb
• Stop exposing build_coverage_regex in lib/api/entities/project.rb
• Write a migration to remove the build_coverage_regex column

cc @jreporter @shampton