Add JSpecify nullability annotations

[mhalbritter]

Loose ends:

• Add dependency management for JSpecify #46776
• Remove varargs checkstyle suppressions (needs release of https://github.com/spring-io/spring-javaformat)
• Add nullability to org.springframework.boot.json.JsonWriter (it's currently @NullUnmarked)
• Rely on org.springframework.core.Nullness to determine if an Actuator endpoint is optional #46593
• Check public API of modules if @Nullable is missing (especially if generics are involved)
• Remove suppression in graphql module once Missing @Nullable annotation on GraphiQlHandler spring-graphql#1276 is fixed
• Review @SuppressWarnings("NullAway") suppressions
• Remove nullAwayVersion and errorProneVersion gradle properties after Upgrade to ErrorProne 2.41.0 spring-gradle-plugins/nullability-plugin#5 and Upgrade to NullAway 0.12.8 spring-gradle-plugins/nullability-plugin#4 is resolved and we're using 0.0.3 of the plugin
• Add ArchUnit / Checkstyle rule to verify that all packages are @NullMarked
• Review org.springframework.boot.hibernate.SpringJtaPlatform#locateTransactionManager and org.springframework.boot.hibernate.SpringJtaPlatform#locateUserTransaction
• Review org.springframework.boot.jpa.EntityManagerFactoryBuilder.Builder#setPackagesToScan
• Review if org.springframework.boot.metrics.autoconfigure.export.properties.PropertiesConfigAdapter.Getter is fine for us.
• Take another look at org.springframework.boot.context.properties.PropertyMapper, especially alwaysApplyingWhenNonNull and whenNonNull.