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-context/src/main/java/org/springframework/instrument/classloading/glassfish/package-info.java

@@ -1,7 +1,4 @@
 /**
  * Support for class instrumentation on GlassFish.
  */
-@NullMarked
 package org.springframework.instrument.classloading.glassfish;
-
-import org.jspecify.annotations.NullMarked;

spring-context/src/main/java/org/springframework/instrument/classloading/jboss/package-info.java

@@ -1,7 +1,4 @@
 /**
  * Support for class instrumentation on JBoss AS 6 and 7.
  */
-@NullMarked
 package org.springframework.instrument.classloading.jboss;
-
-import org.jspecify.annotations.NullMarked;

spring-context/src/main/java/org/springframework/instrument/classloading/package-info.java

@@ -2,7 +2,4 @@
  * Support package for load time weaving based on class loaders,
  * as required by JPA providers (but not JPA-specific).
  */
-@NullMarked
 package org.springframework.instrument.classloading;
-
-import org.jspecify.annotations.NullMarked;

spring-context/src/main/java/org/springframework/instrument/classloading/tomcat/package-info.java

@@ -1,7 +1,4 @@
 /**
  * Support for class instrumentation on Tomcat.
  */
-@NullMarked
 package org.springframework.instrument.classloading.tomcat;
-
-import org.jspecify.annotations.NullMarked;