[LIVY-1012] Use SslContextFactory.Server() instead of SslContextFactory

## What changes were proposed in this pull request?

Use SslContextFactory.Server() instead of SslContextFactory() when constructing the factory.

## How was this patch tested?

With Jetty 9.4.50, we should call SslContextFactory.Server(), instead of SslContextFactory(), to create SslContextFactory. Otherwise we get the following error when using a KeyStore with multiple certificates in it.

```
Exception in thread "main" java.lang.IllegalStateException: KeyStores with multiple certificates are not supported on the base class org.eclipse.jetty.util.ssl.SslContextFactory. (Use org.eclipse.jetty.util.ssl.
SslContextFactory$Server or org.eclipse.jetty.util.ssl.SslContextFactory$Client instead)
at org.eclipse.jetty.util.ssl.SslContextFactory.newSniX509ExtendedKeyManager(SslContextFactory.java:1289)
```

Author: surendralilhore

server/src/main/scala/org/apache/livy/server/WebServer.scala

@@ -49,7 +49,7 @@ class WebServer(livyConf: LivyConf, var host: String, var port: Int) extends Log
       https.setSendServerVersion(livyConf.getBoolean(LivyConf.SEND_SERVER_VERSION))
       https.addCustomizer(new SecureRequestCustomizer())
 
-      val sslContextFactory = new SslContextFactory()
+      val sslContextFactory = new SslContextFactory.Server()
       sslContextFactory.setKeyStorePath(keystore)
 
       val credentialProviderPath = livyConf.get(LivyConf.HADOOP_CREDENTIAL_PROVIDER_PATH)

thriftserver/server/src/main/scala/org/apache/livy/thriftserver/cli/ThriftHttpCLIService.scala

@@ -89,7 +89,7 @@ class ThriftHttpCLIService(
           }
           val keyStorePassword = getKeyStorePassword()
           val keystoreType = livyConf.get(LivyConf.SSL_KEYSTORE_TYPE)
-          val sslContextFactory = new SslContextFactory
+          val sslContextFactory = new SslContextFactory.Server();
           val excludedProtocols = livyConf.get(LivyConf.THRIFT_SSL_PROTOCOL_BLACKLIST).split(",")
           info(s"HTTP Server SSL: adding excluded protocols: $excludedProtocols")
           sslContextFactory.addExcludeProtocols(excludedProtocols: _*)