GH-1192 - Migrate code base to jSpecify for nullness verification.

spring-modulith-api/pom.xml

@@ -17,6 +17,11 @@
 
 	<dependencies>
 
+		<dependency>
+			<groupId>org.jspecify</groupId>
+			<artifactId>jspecify</artifactId>
+		</dependency>
+
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-autoconfigure</artifactId>

spring-modulith-api/src/main/java/org/springframework/modulith/package-info.java

@@ -1,5 +1,5 @@
 /**
  * Core abstractions of Spring Modulith. To be referred to in user applications.
  */
-@org.springframework.lang.NonNullApi
+@org.jspecify.annotations.NullMarked
 package org.springframework.modulith;