GH-8691: Add (S)FTP, SMB aged file filters

Fixes https://github.com/spring-projects/spring-integration/issues/8691

* Remove setAge with TimeUnit
Turned out we don't need anymore since we're using Duration for age in FtpLastModifiedFileListFilter and SftpLastModifiedFileListFilter.
* Add changes to the docs
* Introduce AbstractLastModifiedFileListFilter
* Some code readability improvements
* Make language in the docs more official

src/reference/antora/modules/ROOT/pages/ftp/inbound.adoc

@@ -98,6 +98,11 @@ You should also understand that the FTP inbound channel adapter is a polling con
 Therefore, you must configure a poller (by using either a global default or a local sub-element).
 Once a file has been transferred, a message with a `java.io.File` as its payload is generated and sent to the channel identified by the `channel` attribute.
 
+Starting with version 6.2, you can filter FTP files based on last-modified strategy using `FtpLastModifiedFileListFilter`.
+This filter can be configured with an `age` property so that only files older than this value are passed by the filter.
+The age defaults to 60 seconds, but you should choose an age that is large enough to avoid picking up a file early (due to, say, network glitches).
+Look into its Javadoc for more information.
+
 [[more-on-file-filtering-and-incomplete-files]]
 == More on File Filtering and Incomplete Files
 

src/reference/antora/modules/ROOT/pages/sftp/inbound.adoc

@@ -95,6 +95,11 @@ SFTP inbound channel adapter is a polling consumer.
 Therefore, you must configure a poller (either a global default or a local element).
 Once the file has been transferred to a local directory, a message with `java.io.File` as its payload type is generated and sent to the channel identified by the `channel` attribute.
 
+Starting with version 6.2, you can filter SFTP files based on last-modified strategy using `SftpLastModifiedFileListFilter`.
+This filter can be configured with an `age` property so that only files older than this value are passed by the filter.
+The age defaults to 60 seconds, but you should choose an age that is large enough to avoid picking up a file early (due to, say, network glitches).
+Look into its Javadoc for more information.
+
 [[more-on-file-filtering-and-large-files]]
 == More on File Filtering and Large Files
 

src/reference/antora/modules/ROOT/pages/smb.adoc

@@ -140,6 +140,11 @@ public MessageSource<File> smbMessageSource() {
 
 For XML configuration the `<int-smb:inbound-channel-adapter>` component is provided.
 
+Starting with version 6.2, you can filter SMB files based on last-modified strategy using `SmbLastModifiedFileListFilter`.
+This filter can be configured with an `age` property so that only files older than this value are passed by the filter.
+The age defaults to 60 seconds, but you should choose an age that is large enough to avoid picking up a file early (due to, say, network glitches.
+Look into its Javadoc for more information.
+
 [[configuring-with-the-java-dsl]]
 === Configuring with the Java DSL
 

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -66,3 +66,9 @@ See xref:jdbc/message-store.adoc#jdbc-db-init[Initializing the Database] for mor
 
 A new option `setCreateIndexes(boolean)` has been introduced in `AbstractConfigurableMongoDbMessageStore` to disable the auto indexes creation.
 See xref:mongodb.adoc#mongodb-message-store[MongoDB Message Store] for an example.
+
+[[x6.2-remote-files]]
+=== Remote Files Support Changes
+
+`FtpLastModifiedFileListFilter`, `SftpLastModifiedFileListFilter` and `SmbLastModifiedFileListFilter` have been introduced to allow files filtering based on a last-modified strategy respectively for `FTP`, `SFTP` and `SMB`.
+See xref:ftp/inbound.adoc#ftp-inbound[FTP Inbound Channel Adapter], xref:sftp/inbound.adoc#sftp-inbound[SFTP Inbound Channel Adapter], and xref:smb.adoc#smb-inbound[SMB Inbound Channel Adapter] for more information.