SpringBoot Upgrade to spring-asciidoctor-extensions 0.5.0

https://github.com/spring-projects/spring-boot/issues/23605 seems to be meant to upgrade to spring-asciidoctor-extensions 0.5.0, but it has upgraded to spring-asciidoctor-extensions 0.5.0-SNAPSHOT somehow.

This PR upgrades to spring-asciidoctor-extensions 0.5.0.

Comment From: snicoll

Good catch, thanks again @izeye