Migrate away from deprecated Gradle plugin convention

[ciscoo]

spring-javaformat/spring-javaformat-gradle/spring-javaformat-gradle-plugin/src/main/java/io/spring/javaformat/gradle/SpringJavaFormatPlugin.java

Line 59 in adf3ecf

this.project.getConvention()

The above raises a warning starting with Gradle 8.2 as conventions will be removed in Gradle 9.

https://docs.gradle.org/8.2-rc-1/userguide/upgrading_version_8.html#deprecated_access_to_conventions