Revert express upgrade (#467)

`@cds/cds` does not yet support `express` in version 5. The previous
dependabot upgrade broke the BTP integration tests with the following
error. Switching back to version 4.

```
While resolving: @sap/[email protected]

[2025-04-09T06:46:54.506Z] npm error Found: [email protected]
[2025-04-09T06:46:54.506Z] npm error node_modules/express
[2025-04-09T06:46:54.506Z] npm error   express@"^5" from the root project
[2025-04-09T06:46:54.506Z] npm error   peer express@">=4" from @sap/[email protected]
[2025-04-09T06:46:54.506Z] npm error   node_modules/@sap/cds/node_modules/@sap/cds-fiori
[2025-04-09T06:46:54.506Z] npm error     @sap/cds-fiori@"^1" from @sap/[email protected]
[2025-04-09T06:46:54.506Z] npm error
[2025-04-09T06:46:54.506Z] npm error Could not resolve dependency:
[2025-04-09T06:46:54.506Z] npm error peerOptional express@"^4" from @sap/[email protected]
[2025-04-09T06:46:54.506Z] npm error node_modules/@sap/cds
[2025-04-09T06:46:54.506Z] npm error   @sap/cds@"^8" from the root project
[2025-04-09T06:46:54.506Z] npm error   peer @sap/cds@">=7.9" from @cap-js/[email protected]
[2025-04-09T06:46:54.506Z] npm error   node_modules/@cap-js/db-service
[2025-04-09T06:46:54.506Z] npm error     @cap-js/db-service@"^1.19.0" from @cap-js/[email protected]
[2025-04-09T06:46:54.506Z] npm error     node_modules/@cap-js/hana
[2025-04-09T06:46:54.506Z] npm error       @cap-js/hana@"^1" from the root project
[2025-04-09T06:46:54.506Z] npm error     1 more (@cap-js/sqlite)
[2025-04-09T06:46:54.506Z] npm error   3 more (@cap-js/hana, @cap-js/sqlite, @sap/cds-fiori)
[2025-04-09T06:46:54.506Z] npm error
[2025-04-09T06:46:54.506Z] npm error Conflicting peer dependency: [email protected]
[2025-04-09T06:46:54.506Z] npm error node_modules/express
[2025-04-09T06:46:54.506Z] npm error   peerOptional express@"^4" from @sap/[email protected]
[2025-04-09T06:46:54.506Z] npm error   node_modules/@sap/cds
[2025-04-09T06:46:54.506Z] npm error     @sap/cds@"^8" from the root project
[2025-04-09T06:46:54.506Z] npm error     peer @sap/cds@">=7.9" from @cap-js/[email protected]
[2025-04-09T06:46:54.506Z] npm error     node_modules/@cap-js/db-service
[2025-04-09T06:46:54.506Z] npm error       @cap-js/db-service@"^1.19.0" from @cap-js/[email protected]
[2025-04-09T06:46:54.506Z] npm error       node_modules/@cap-js/hana
[2025-04-09T06:46:54.506Z] npm error         @cap-js/hana@"^1" from the root project
[2025-04-09T06:46:54.506Z] npm error       1 more (@cap-js/sqlite)
[2025-04-09T06:46:54.506Z] npm error     3 more (@cap-js/hana, @cap-js/sqlite, @sap/cds-fiori)
```

Author: StefanHenke