Update for Testcontainers 2.0

When selecting "Testcontainers" on start.spring.io with Boot 4.0.0-SNAPSHOT, I get this on `gradle build`:

```
Execution failed for task ':compileTestJava'.
> Could not resolve all files for configuration ':testCompileClasspath'.
   > Could not find org.testcontainers:junit-jupiter:.
     Required by:
         root project 'demo'
```