[SPARK-25578][BUILD] Update to Scala 2.12.7

## What changes were proposed in this pull request?

Update to Scala 2.12.7. See https://issues.apache.org/jira/browse/SPARK-25578 for why.

## How was this patch tested?

Existing tests.

Closes #22600 from srowen/SPARK-25578.

Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
(cherry picked from commit 5114db5)
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: srowen

pom.xml

@@ -2760,7 +2760,7 @@
     <profile>
       <id>scala-2.12</id>
       <properties>
-        <scala.version>2.12.6</scala.version>
+        <scala.version>2.12.7</scala.version>
         <scala.binary.version>2.12</scala.binary.version>
       </properties>
       <build>