Scope search pattern to ff directories

What

Update search pattern to specifically look in feature flag directories

• Converts general search through all files to strictly search through config/feature_flags directories (ee included)
• f:\.yml$ to file:^(config/feature_flags/|ee/config/feature_flags/)

Why

Feature deletion was failing as non feature yaml files were being returned

• Scope based on https://docs.gitlab.com/ee/user/search/exact_code_search.html

Notes

Tested using an API client and sending the following for input:

• ee: compliance_standards_adherence_csv_export file:^(config/feature_flags/|ee/config/feature_flags/)
• saas: add_branch_rule file:^(config/feature_flags/|ee/config/feature_flags/)

Related to #174 (closed)