[KYUUBI #1112] Upgrade scala to 2.12.15

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### _Why are the changes needed?_
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Close #1112.

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [ ] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #1120 from byyue/feature-1112.

Closes #1112

9bb10858 [Brian Yue] [FEATURE] [Dependency] Upgrade scala to 2.14.15

Authored-by: Brian Yue <code.byyue@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>

.travis.yml

@@ -25,7 +25,7 @@ branches:
 dist: bionic
 language: scala
 scala:
-  - 2.12.14
+  - 2.12.15
 jdk:
   - openjdk8
 

dev/dependencyList

@@ -72,7 +72,7 @@ metrics-json/4.1.1//metrics-json-4.1.1.jar
 metrics-jvm/4.1.1//metrics-jvm-4.1.1.jar
 osgi-resource-locator/1.0.3//osgi-resource-locator-1.0.3.jar
 paranamer/2.8//paranamer-2.8.jar
-scala-library/2.12.14//scala-library-2.12.14.jar
+scala-library/2.12.15//scala-library-2.12.15.jar
 scopt_2.12/4.0.1//scopt_2.12-4.0.1.jar
 simpleclient/0.10.0//simpleclient-0.10.0.jar
 simpleclient_common/0.10.0//simpleclient_common-0.10.0.jar

pom.xml

@@ -84,7 +84,7 @@
         <maven.version>3.6.3</maven.version>
         <maven.compiler.source>${java.version}</maven.compiler.source>
         <maven.compiler.target>${java.version}</maven.compiler.target>
-        <scala.version>2.12.14</scala.version>
+        <scala.version>2.12.15</scala.version>
         <scala.binary.version>2.12</scala.binary.version>
 
         <antlr4.version>4.8</antlr4.version>
@@ -149,7 +149,7 @@
         <maven.plugin.jar.version>3.2.0</maven.plugin.jar.version>
         <maven.plugin.scalastyle.version>1.0.0</maven.plugin.scalastyle.version>
         <maven.plugin.shade.version>3.2.4</maven.plugin.shade.version>
-        <maven.plugin.silencer.version>1.7.5</maven.plugin.silencer.version>
+        <maven.plugin.silencer.version>1.7.6</maven.plugin.silencer.version>
         <maven.plugin.source.version>3.2.1</maven.plugin.source.version>
         <maven.plugin.rat.version>0.13</maven.plugin.rat.version>