Rename vulnerability_finding's fingerprints to signatures

changelogs/unreleased/rename-vuln-fingerprints.yml

db/migrate/20210329191850_add_finding_signature_table.rb

..._migrate/20210330130420_drop_finding_fingerprint_table.rb

db/schema_migrations/20210329191850

db/schema_migrations/20210330130420

db/structure.sql

ee/app/models/vulnerabilities/finding.rb

ee/app/models/vulnerabilities/finding_signature.rb

ee/app/services/security/store_report_service.rb

ee/lib/gitlab/ci/parsers/security/common.rb

ee/lib/gitlab/ci/reports/security/finding.rb

ee/lib/gitlab/ci/reports/security/finding_signature.rb

ee/spec/factories/vulnerabilities/finding_signatures.rb

ee/spec/lib/gitlab/ci/parsers/security/common_spec.rb

.../lib/gitlab/ci/reports/security/finding_signature_spec.rb

ee/spec/services/security/store_report_service_spec.rb

spec/fixtures/security_reports/master/gl-sast-report.json