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-smb/src/test/java/org/springframework/integration/smb/filters/SmbLastModifiedFileListFilterTests.java

@@ -0,0 +1,55 @@
+/*
+ * 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.smb.filters;
+
+import java.time.Duration;
+import java.time.Instant;
+
+import jcifs.smb.SmbFile;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * @author Adama Sorho
+ * @author Artem Bilan
+ *
+ * @since 6.2
+ */
+public class SmbLastModifiedFileListFilterTests {
+
+	@Test
+	public void testAge() {
+		SmbLastModifiedFileListFilter filter = new SmbLastModifiedFileListFilter();
+		filter.setAge(80);
+		SmbFile smbFile1 = mock(SmbFile.class);
+		when(smbFile1.getLastModified()).thenReturn(System.currentTimeMillis());
+		SmbFile smbFile2 = mock(SmbFile.class);
+		when(smbFile2.getLastModified()).thenReturn(System.currentTimeMillis());
+		SmbFile smbFile3 = mock(SmbFile.class);
+
+		when(smbFile3.getLastModified())
+				.thenReturn(Instant.now().minus(Duration.ofDays(1)).toEpochMilli());
+		SmbFile[] files = new SmbFile[] {smbFile1, smbFile2, smbFile3};
+		assertThat(filter.filterFiles(files)).hasSize(1);
+		assertThat(filter.accept(smbFile1)).isFalse();
+		assertThat(filter.accept(smbFile3)).isTrue();
+	}
+
+}