Remove duplicate source files from old ApiKeys/, Authentication/, and
Authorization/ folders that were already migrated to Common/Security/.
Update test namespace usings to match. Pin EFC Relational 10.0.5 to
resolve MSB3277 version conflict with Npgsql's transitive dependency.
Add LangVersion and TreatWarningsAsErrors to AppHost and ServiceDefaults.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
