[minor][docs] add JDK baseline version and remove useless code (#4238)

Author: Hisoka-X

.gitignore

@@ -2,7 +2,6 @@
 scalastyle-output.xml
 .classpath
 .idea/*
-!.idea/vcs.xml
 .metadata
 .settings
 .project

docs/content.zh/docs/developer-guide/contribute-to-flink-cdc.md

@@ -48,6 +48,8 @@ Flink CDC 项目通过众多贡献者的代码贡献来维护，改进和拓展
 
 在 Flink CDC 社区可以自由的在任何时间提出自己的问题，通过社区 Dev 邮件列表进行交流或在任何感兴趣的 issue 下评论和讨论。
 
+Flink CDC 使用 JDK 11 作为基础版本，请确保您的开发环境已经正确配置。
+
 如果您想要为 Flink CDC 贡献代码，可以通过如下的方式。
 
 1. 首先在 [Flink jira](https://issues.apache.org/jira/projects/FLINK/issues) 的想要负责的 issue

docs/content/docs/developer-guide/contribute-to-flink-cdc.md

@@ -51,6 +51,8 @@ Flink CDC is maintained, improved, and extended by code contributions of volunte
 
 Please feel free to ask questions at any time. Either send a mail to the Dev mailing list or comment on the issue you are working on.
 
+Flink CDC uses JDK 11 as the baseline version. Please make sure your development environment is set up correctly.
+
 If you would like to contribute to Flink CDC, you could raise it as follows.
 
 1. Left comment under the issue that you want to take.  (It's better to explain your understanding of the issue, and 

flink-cdc-e2e-tests/flink-cdc-source-e2e-tests/src/test/java/org/apache/flink/cdc/connectors/tests/MongoE2eITCase.java

@@ -43,7 +43,6 @@
 import java.time.Duration;
 import java.util.Arrays;
 import java.util.List;
-import java.util.Objects;
 import java.util.Random;
 import java.util.stream.Stream;
 
@@ -65,16 +64,7 @@ class MongoE2eITCase extends FlinkContainerTestEnvironment {
 
     private MongoClient mongoClient;
 
-    public static String getMongoVersion() {
-        String specifiedMongoVersion = System.getProperty("specifiedMongoVersion");
-        if (Objects.isNull(specifiedMongoVersion)) {
-            throw new IllegalArgumentException(
-                    "No MongoDB version specified to run this test. Please use -DspecifiedMongoVersion to pass one.");
-        }
-        return specifiedMongoVersion;
-    }
-
-    void setup(String mongoVersion, boolean parallelismSnapshot, boolean scanFullChangelog) {
+    void setup(String mongoVersion, boolean scanFullChangelog) {
         container =
                 new MongoDBContainer("mongo:" + mongoVersion)
                         .withSharding()
@@ -123,7 +113,7 @@ public void after() {
     })
     void testMongoDbCDC(String mongoVersion, boolean parallelismSnapshot, boolean scanFullChangelog)
             throws Exception {
-        setup(mongoVersion, parallelismSnapshot, scanFullChangelog);
+        setup(mongoVersion, scanFullChangelog);
         String dbName =
                 container.executeCommandFileInDatabase(
                         "mongo_inventory",