Replace deprecated Project.task()

[izeye]

This PR replaces deprecated Project.task() with tasks.register().

I just left the aggregatedJavadoc task as-is as the simple replacement didn't work with the following failure:

spring-boot/spring-boot-project/spring-boot-docs/build.gradle

Line 207 in e87d5d7

task aggregatedJavadoc(type: Javadoc) {

* What went wrong:
A problem occurred evaluating project ':spring-boot-project:spring-boot-docs'.
> Could not create task ':spring-boot-project:spring-boot-docs:aggregatedJavadoc'.
   > Gradle#projectsEvaluated(Closure) on build 'spring-boot-build' cannot be executed in the current context.

See https://docs.gradle.org/current/userguide/upgrading_version_8.html#source_level_deprecation_of_project_task_methods