GH-9129: SFTP: Remove extra / for the file_remoteDirectory header

Fixes: #9129

If `.remoteDirectory("/sftpSource")`, then `FileHeaders.REMOTE_DIRECTORY` is `//sftpSource`

* Fix `AbstractInboundFileSynchronizer` to not add `/` if one is already present in the beginning of the `remoteDirectoryPath`

**Auto-cherry-pick to `6.2.x` & `6.1.x`**

spring-integration-file/src/main/java/org/springframework/integration/file/remote/synchronizer/AbstractInboundFileSynchronizer.java

@@ -502,7 +502,10 @@ public abstract class AbstractInboundFileSynchronizer<F>
 				try {
 					String remoteFileMetadata =
 							new URI(protocol(), null, host, Integer.parseInt(port),
-									'/' + remoteDirectoryPath, null, remoteFileName)
+									remoteDirectoryPath.charAt(0) == '/'
+											? remoteDirectoryPath :
+											'/' + remoteDirectoryPath,
+									null, remoteFileName)
 									.toString();
 					this.remoteFileMetadataStore.put(buildMetadataKey(localFile), remoteFileMetadata);
 				}