Disallow @org.jetbrains.annotations.Nullable imports

This commit adds a checkstyle rule that rejects
`@org.jetbrains.annotations.Nullable` imports in the source code.

See gh-35114

Author: bclozel

src/checkstyle/checkstyle.xml

@@ -107,7 +107,7 @@
 			<property name="id" value="bannedImports"/>
 			<property name="regexp" value="true"/>
 			<property name="illegalClasses"
-				value="^reactor\.core\.support\.Assert,^org\.slf4j\.LoggerFactory,^(?!org\.jspecify|\.annotations).*(NonNull|Nullable)$"/>
+				value="^reactor\.core\.support\.Assert,^org\.slf4j\.LoggerFactory,^(?!org\.jspecify|\.annotations).*(NonNull|Nullable),^org\.jetbrains\.annotations\.Nullable$"/>
 		</module>
 		<module name="com.puppycrawl.tools.checkstyle.checks.imports.IllegalImportCheck">
 			<property name="id" value="bannedJUnit3Imports"/>