Consistently document the minimum supported versions of Gradle

[wilkinsona]

At the time of writing, https://docs.spring.io/spring-boot/gradle-plugin/index.html incorrectly states that "Gradle 7.x (7.5 or later) or 8.x" is supported while https://docs.spring.io/spring-boot/system-requirements.html correctly states that it's "Gradle 7.x (7.6.4 or later) or 8.x (8.4 or later)" that we support.