SpringBoot Remove calls to deprecated Project.getBuildDir()

> Task :buildSrc:compileJava
Note: ~/Projects/spring-boot/3.2.x/buildSrc/src/main/java/org/springframework/boot/build/autoconfigure/AutoConfigurationPlugin.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details

Comment From: mhalbritter

There are references to $buildDir in our Gradle files. This will be fixed in #42739.