Fix the rubygems extraction service to not create subtransactions

Replace a `.safe_find_or_create_by!` with `.find_or_create_by!` to avoid
creating conditions where a subtransaction could be created.

Changelog: fixed

app/services/packages/rubygems/metadata_extraction_service.rb

@@ -49,7 +49,11 @@ module Packages
       # rubocop:enable Metrics/CyclomaticComplexity
 
       def metadatum
-        Packages::Rubygems::Metadatum.safe_find_or_create_by!(package: package)
+        # safe_find_or_create_by! was originally called here.
+        # We merely switched to `find_or_create_by!`
+        # rubocop: disable CodeReuse/ActiveRecord
+        Packages::Rubygems::Metadatum.find_or_create_by!(package: package)
+        # rubocop: enable CodeReuse/ActiveRecord
       end
     end
   end

spec/services/packages/rubygems/metadata_extraction_service_spec.rb

@@ -46,5 +46,13 @@ RSpec.describe Packages::Rubygems::MetadataExtractionService do
       expect(metadata.requirements).to eq(gemspec.requirements.to_json)
       expect(metadata.rubygems_version).to eq(gemspec.rubygems_version)
     end
+
+    context 'with an existing metadatum' do
+      let_it_be(:metadatum) { create(:rubygems_metadatum, package: package) }
+
+      it 'updates it' do
+        expect { subject }.not_to change { Packages::Rubygems::Metadatum.count }
+      end
+    end
   end
 end