feat: spring: update deprecated org.springframework.lang.Nullable for

Spring Boot 4

- Update import mapping for Nullable to
org.jspecify.annotations.Nullable when useSpringBoot4 is enabled.

Author: tsungyuanyeh

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/SpringCodegen.java

@@ -551,7 +551,7 @@ public void processOpts() {
         importMapping.put("JsonDeserialize", (useJackson3 ? JACKSON3_PACKAGE : JACKSON2_PACKAGE) + ".databind.annotation.JsonDeserialize");
 
         typeMapping.put("file", "org.springframework.core.io.Resource");
-        importMapping.put("Nullable", "org.springframework.lang.Nullable");
+        importMapping.put("Nullable", isUseSpringBoot4() ? "org.jspecify.annotations.Nullable" : "org.springframework.lang.Nullable");
         importMapping.put("org.springframework.core.io.Resource", "org.springframework.core.io.Resource");
         importMapping.put("DateTimeFormat", "org.springframework.format.annotation.DateTimeFormat");
         importMapping.put("ParameterObject", "org.springdoc.api.annotations.ParameterObject");