Upgrade to Spring Boot 2.4.3

[sdeleuze]

Done:

• Switch ReactiveAdapterRegistry to build time initialization (see related commit on Spring Framework).

TODO:

• Try to see if that's possible to enable Kotlin reflection in DefaultParameterNameDiscoverer or not.
• Introduce a NativeDetector utility class with -H:+InlineBeforeAnalysis enabled by default

[sdeleuze]

Native image creation with Boot 2.4.2 has been broken due to spring-projects/spring-boot#24192, we need a buildpack update that only prepends BOOT-INF/lib if the entry doesn't already start with that, as pointed out by @wilkinsona.

[sdeleuze]

Asap released we should upgrade to Spring Framework 5.3.4 and leverage the new NativeDetector, see this related commit. NativeDetector should be configured init at build time and the build time init config for DefaultAopProxyFactory, AbstractAutowireCapableBeanFactory, ConfigurationClassPostProcessor, AbstractApplicationContext, DefaultParameterNameDiscoverer and SerializableTypeWrapper should probably be removed.