fix: remove dead SchemaMigrationService and unused Migration entity - #1377

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4, pnpm/action-setup@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

'metadata' is assigned a value but never used. Allowed unused args must match /^_/u

'metadata' is assigned a value but never used. Allowed unused args must match /^_/u

'queryRunner' is defined but never used. Allowed unused args must match /^_/u

'validateWebhookUrl' is defined but never used. Allowed unused vars must match /^_/u

'Tenant' is defined but never used. Allowed unused vars must match /^_/u