Document support for Java 18

Closes gh-30548

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

@@ -1,6 +1,6 @@
 [[getting-started.system-requirements]]
 == System Requirements
-Spring Boot {spring-boot-version} requires https://www.java.com[Java 8] and is compatible up to and including Java 17.
+Spring Boot {spring-boot-version} requires https://www.java.com[Java 8] and is compatible up to and including Java 18.
 {spring-framework-docs}/[Spring Framework {spring-framework-version}] or above is also required.
 
 Explicit build support is provided for the following build tools: