When running on Java 11 (where `@PostConstruct` is no longer part of
the JRE) and without a dependency on jakarta-annotation-api,
`@PostContruct` annotions are silently dropped. This leads to obscure and
hard-to-track down changes in the behaviour of our auto-configuration
as the `@PostConstruct`-annotated methods are not invoked.

To allow users to run on Java 11 without having jakarta-annotation-api
on the classpath, this commit removes use of `@PostConstruct` from main
code. A Checkstyle rule has also been added to prevent its usage in
main code from being reintroduced.

Closes gh-23723