08:04:28.877 [main] ERROR o.a.c.h.Http11NioProtocol - [log,173] - The upgrade handler [org.apache.coyote.http2.Http2Protocol] for [h2] only supports upgrade via ALPN but has been configured for the ["https-jsse-nio-443"] connector that does not support ALPN.
The spring-boot-webflux-starter is using by default Reactor Netty as a server. Reactor Netty can be configured for HTTP/2 using the JDK support with JDK 9 or later. For JDK 8 environments, or for optimal runtime performance, this server also supports HTTP/2 with native libraries. To enable that, your application needs to have an additional dependency.
Spring Boot manages the version for the io.netty:netty-tcnative-boringssl-static "uber jar", containing native libraries for all platforms. Developers can choose to import only the required dependencies using a classifier (see the Netty official documentation).