As htttpclient 4.x is not supported anymore by `RestTemplate`, this
commit changes such dependencies to httpclient5 instead. In some cases,
the httpclient 4.x was transitively brought by a non-Spring dependency.
See gh-32461
This commit adds the `AutoConfigurationImportsAnnotationProcessor` to
the `spring-boot-autoconfigure-processor` annotation processor
module. When added to a project build, the annotation processor will
generate the
`org.springframework.boot.autoconfigure.AutoConfiguration.imports`
file automatically from `@AutoConfiguration`-annotated classes. It
also applies the annotation processor to the Spring Boot build.
Closes gh-31228
