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>

plugins/code-coverage-backend/src/service/CodeCoverageDatabase.test.ts

@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import { SingleConnectionDatabaseManager } from '@backstage/backend-common';
+import { DatabaseManager } from '@backstage/backend-common';
 import { stringifyEntityRef } from '@backstage/catalog-model';
 import { ConfigReader } from '@backstage/config';
 import {
@@ -22,7 +22,7 @@ import {
 } from './CodeCoverageDatabase';
 import { JsonCodeCoverage } from './types';
 
-const db = SingleConnectionDatabaseManager.fromConfig(
+const db = DatabaseManager.fromConfig(
   new ConfigReader({
     backend: {
       database: {

plugins/code-coverage-backend/src/service/router.test.ts

@@ -20,14 +20,14 @@ import {
   getVoidLogger,
   PluginDatabaseManager,
   PluginEndpointDiscovery,
-  SingleConnectionDatabaseManager,
+  DatabaseManager,
   UrlReaders,
 } from '@backstage/backend-common';
 import { ConfigReader } from '@backstage/config';
 import { createRouter } from './router';
 
 function createDatabase(): PluginDatabaseManager {
-  return SingleConnectionDatabaseManager.fromConfig(
+  return DatabaseManager.fromConfig(
     new ConfigReader({
       backend: {
         database: {