[modernize] Use standard charsets

unfortunately if > jdk 16 uses with current project needs, the detection can be a bit off.  So this is not entirely ideal until we are on higher jdks and stop using static for searching when we already have it.  For now this is better.

Author: hazendaz

src/main/java/org/apache/ibatis/io/DefaultVFS.java

@@ -22,6 +22,7 @@
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.UnsupportedEncodingException;
+import java.nio.charset.StandardCharsets;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLEncoder;
@@ -269,7 +270,7 @@ protected URL findJarForResource(URL url) throws MalformedURLException {
         // File name might be URL-encoded
         if (!file.exists()) {
           try {
-            file = new File(URLEncoder.encode(jarUrl.toString(), "UTF-8"));
+            file = new File(URLEncoder.encode(jarUrl.toString(), StandardCharsets.UTF_8.name()));
           } catch (UnsupportedEncodingException e) {
             throw new RuntimeException("Unsupported encoding?  UTF-8?  That's impossible.");
           }

src/test/java/org/apache/ibatis/io/ExternalResourcesTest.java

@@ -21,6 +21,7 @@
 import java.io.FileNotFoundException;
 import java.io.FileWriter;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;