Remove warning concerning annotation processor sources

When running Gradle with --warning-mode all, we currently get this warning:

Configure project :bramble-core The CompileOptions.annotationProcessorGeneratedSourcesDirectory property has been deprecated. This is scheduled to be removed in Gradle 8.0. Please use the generatedSourceOutputDirectory property instead. See https://docs.gradle.org/7.3.3/dsl/org.gradle.api.tasks.compile.CompileOptions.html#org.gradle.api.tasks.compile.CompileOptions:annotationProcessorGeneratedSourcesDirectory for more details. at dagger_aws623iifvykitf9kogs8i23w_run_closure2_closure4.doCall(/home/z/gitlab/briar/briar/dagger.gradle:9)