AcquireTimerJobsRunnable is not awaiting termination of its ExecutorService

**Describe the bug**
`AcquireTimerJobsRunnable#run` is not awaiting termination of its `ExecutorService` (only triggering a shutdown).

**Expected behavior**
`AcquireTimerJobsRunnable#run` should await the termination of its `ExecutorService` similarly to `DefaultAsyncTaskExecutor#shutdown`. All other similar runners don't use `ExecutorService` at all and really "stop" when they are meant to do so.
This causes intermittent test failures and application shutdown issues due to still holding connections to the DB when there is still a running task in the executor service.

**Additional context**
Flowable version: 7.0.0.M1, standalone usage. Database: Postgres 14.