Fix outdated Spring Batch Javadoc links in reference docs

Update reference documentation links to point to the Spring Batch 6 API location under /reference/api and remove legacy /docs/current/api and /apidocs paths.

Resolves #5290

Signed-off-by: HongYeseul <yeseul.dev@gmail.com>

Author: HongYeseul

spring-batch-docs/modules/ROOT/pages/job/advanced-meta-data.adoc

@@ -49,7 +49,7 @@ need to populate it manually with the jobs that you want to operate through the
 
 Most of the methods on `JobOperator` are
 self-explanatory, and you can find more detailed explanations in the
-https://docs.spring.io/spring-batch/docs/current/api/org/springframework/batch/core/launch/JobOperator.html[Javadoc of the interface]. However, the
+https://docs.spring.io/spring-batch/reference/api/org/springframework/batch/core/launch/JobOperator.html[Javadoc of the interface]. However, the
 `startNextInstance` method is worth noting. This
 method always starts a new instance of a `Job`.
 This can be extremely useful if there are serious issues in a

spring-batch-docs/modules/ROOT/pages/spring-batch-integration/externalizing-execution.adoc

@@ -327,7 +327,7 @@ configured `SimpleChunkProcessor`, which is where you would provide a reference
 when it receives chunks from the manager.
 
 For more information, see the section of the "`Scalability`" chapter on
-link:$$https://docs.spring.io/spring-batch/docs/current/reference/html/scalability.html#remoteChunking$$[Remote Chunking].
+link:$$https://docs.spring.io/spring-batch/reference/scalability.html#remoteChunking[Remote Chunking].
 
 Starting from version 4.1, Spring Batch Integration introduces the `@EnableBatchIntegration`
 annotation that can be used to simplify a remote chunking setup. This annotation provides

spring-batch-docs/modules/ROOT/pages/testing.adoc

@@ -344,4 +344,4 @@ public void testAfterStep() {
 
 The preceding method for creating a simple `StepExecution` is only one convenience method
 available within the factory. You can find a full method listing in its
-link:$$http://docs.spring.io/spring-batch/apidocs/org/springframework/batch/test/MetaDataInstanceFactory.html$$[Javadoc].
+link:$$https://docs.spring.io/spring-batch/reference/api/org/springframework/batch/test/MetaDataInstanceFactory.html$$[Javadoc].

spring-batch-docs/src/main/javadoc/overview.html

@@ -6,7 +6,7 @@
 <div id="overviewBody">
 	<p>
 		For further API reference and developer documentation, see the
-		<a href="https://docs.spring.io/spring-batch/docs/current/reference/html/index.html" target="_top">
+		<a href="https://docs.spring.io/spring-batch/reference/index.html" target="_top">
 		Spring Batch reference documentation</a>.
 		That documentation contains more detailed, developer-targeted
 		descriptions, with conceptual overviews, definitions of terms,