Leverage @NullMarked instead of package configuration

This commit replaces NullAway package configuration by package-level
`@NullMarked` detection by configuring `NullAway:AnnotatedPackages` to
an empty string.

NullAway configuration should be refined to use a proper flag instead
once uber/NullAway#574 is fixed.

See gh-28797

spring-core/src/main/java/org/springframework/aot/nativex/feature/package-info.java

@@ -1,7 +1,4 @@
 /**
  * GraalVM native image features, not part of Spring Framework public API.
  */
-@NullMarked
 package org.springframework.aot.nativex.feature;
-
-import org.jspecify.annotations.NullMarked;

spring-core/src/main/java/org/springframework/aot/nativex/substitution/package-info.java

@@ -1,7 +1,4 @@
 /**
  * GraalVM native image substitutions, not part of Spring Framework public API.
  */
-@NullMarked
 package org.springframework.aot.nativex.substitution;
-
-import org.jspecify.annotations.NullMarked;