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

spring-integration-ftp/src/main/java/org/springframework/integration/ftp/filters/FtpLastModifiedFileListFilter.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.ftp.filters;
+
+import java.time.Duration;
+import java.time.Instant;
+import java.util.function.Consumer;
+
+import org.apache.commons.net.ftp.FTPFile;
+
+import org.springframework.integration.file.filters.AbstractLastModifiedFileListFilter;
+
+/**
+ * The {@link AbstractLastModifiedFileListFilter} implementation to filter those files which
+ * {@link FTPFile#getTimestampInstant()} is less than the age in comparison with the {@link Instant#now()}.
+ * When discardCallback {@link #addDiscardCallback(Consumer)} is provided, it called for all the rejected files.
+ *
+ * @author Adama Sorho
+ * @author Artem Bilan
+ *
+ * @since 6.2
+ */
+public class FtpLastModifiedFileListFilter extends AbstractLastModifiedFileListFilter<FTPFile> {
+
+	public FtpLastModifiedFileListFilter() {
+		super();
+	}
+
+	/**
+	 * Construct a {@link FtpLastModifiedFileListFilter} instance with provided age.
+	 * Defaults to 60 seconds.
+	 * @param age the age in seconds.
+	 */
+	public FtpLastModifiedFileListFilter(long age) {
+		this(Duration.ofSeconds(age));
+	}
+
+	/**
+	 * Construct a {@link FtpLastModifiedFileListFilter} instance with provided age.
+	 * Defaults to 60 seconds.
+	 * @param age the Duration
+	 */
+	public FtpLastModifiedFileListFilter(Duration age) {
+		super(age);
+	}
+
+	@Override
+	protected Instant getLastModified(FTPFile remoteFile) {
+		return remoteFile.getTimestampInstant();
+	}
+
+}