refactor: migrate from deprecated database manager

Changes:
- Swaps out `SingleConnectionDatabaseManager` to `DatabaseManager`
  across the repo.
- Updates `backend-test-utils` to generate test plugin names prefixed
  with db to satisfy plugin naming constraint, e.g. 0 becomes db0.

Signed-off-by: Minn Soe <contributions@minn.io>

packages/backend-common/config.d.ts

@@ -14,16 +14,6 @@
  * limitations under the License.
  */
 
-export type PluginDatabaseConfig = {
-  /** Database client to use. */
-  client?: 'sqlite3' | 'pg';
-  /**
-   * Database connection to use.
-   * @secret
-   */
-  connection?: string | object;
-};
-
 export interface Config {
   app: {
     baseUrl: string; // defined in core, but repeated here without doc