Fix supported java version range in documentation

Closes gh-17476

Author: snicoll

spring-boot-project/spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -39,7 +39,7 @@ configuration).
 [[getting-started-system-requirements]]
 == System Requirements
 Spring Boot {spring-boot-version} requires https://www.java.com[Java 8] and is compatible
-up to Java 11 (included). {spring-reference}[Spring Framework {spring-framework-version}]
+up to Java 12 (included). {spring-reference}[Spring Framework {spring-framework-version}]
 or above is also required.
 
 Explicit build support is provided for the following build tools: