THRIFT-5570: update java lib document about gradle usage (#2583)

* update java doc about gradle usage
* update .gitignore
diff --git a/doc/install/debian.md b/doc/install/debian.md
index f1edce5..9be7265 100644
--- a/doc/install/debian.md
+++ b/doc/install/debian.md
@@ -18,7 +18,7 @@
 If you would like to build Apache Thrift libraries for other programming languages you may need to install additional packages. The following languages require the specified additional packages:
 
  * Java
-	* packages: gradle 
+	* packages: gradle (version 6.9.2)
 	* You will also need Java JDK v1.8 or higher. Type **javac** to see a list of available packages, pick the one you prefer and **apt-get install** it (e.g. default-jdk).
  * Ruby
 	* ruby-full ruby-dev ruby-rspec rake rubygems bundler
@@ -51,7 +51,7 @@
  * dart & pub
     * https://www.dartlang.org/install/linux
     * https://www.dartlang.org/tools/pub/installing
-	
+
 
 ## Additional reading