Throw hard failure with config first bootstrap mode during AOT processing

When an application uses config first bootstrap (by configuring spring.config.use-legacy-processing=true or using the spring-cloud-starter-bootstrap dependency), AOT processing is not supported.

While this limitation is documented, an application that transitively depends on the starter will see the following behavior:

• AOT processing succeeds and generates source files
• the application fails at startup both in native and JVM modes, as bean definitions for the application and auto-configurations were not parsed.

Adding an AOT processor in the starter (and declaring it in aot.factories) that throws a hard failure during AOT processing, describing the problem, could help diagnose such issues.