Use `#find_issues_by_hash` in lib/gitlab_quality/test_tooling/report/relate_failure_issue.rb and lib/gitlab_quality/test_tooling/report/slow_test_issue.rb

We should align the search-strategy for lib/gitlab_quality/test_tooling/report/relate_failure_issue.rb and lib/gitlab_quality/test_tooling/report/slow_test_issue.rb to use #find_issues_by_hash, which is already used successfully in lib/gitlab_quality/test_tooling/report/flaky_test_issue.rb.