[SPARK-26266][BUILD] Update to Scala 2.12.8 (branch-2.4)

## What changes were proposed in this pull request?

Back-port of #23218 ; updates Scala 2.12 build to 2.12.8

## How was this patch tested?

Existing tests.

Closes #23264 from srowen/SPARK-26266.2.

Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: srowen

pom.xml

@@ -2767,7 +2767,7 @@
     <profile>
       <id>scala-2.12</id>
       <properties>
-        <scala.version>2.12.7</scala.version>
+        <scala.version>2.12.8</scala.version>
         <scala.binary.version>2.12</scala.binary.version>
       </properties>
       <build>