[SPARK-36759][BUILD][FOLLOWUP] Update version in scala-2.12 profile and doc

### What changes were proposed in this pull request?

This is a follow-up to fix the leftover during switching the Scala version.

### Why are the changes needed?

This should be consistent.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

This is not tested by UT. We need to check manually. There is no more `2.12.14`.
```
$ git grep 2.12.14
R/pkg/tests/fulltests/test_sparkSQL.R:               c(as.Date("2012-12-14"), as.Date("2013-12-15"), as.Date("2014-12-16")))
data/mllib/ridge-data/lpsa.data:3.5307626,0.987291634724086 -0.36279314978779 -0.922212414640967 0.232904453212813 -0.522940888712441 1.79270085261407 0.342627053981254 1.26288870310799
sql/hive/src/test/resources/data/files/over10k:-3|454|65705|4294967468|62.12|14.32|true|mike white|2013-03-01 09:11:58.703087|40.18|joggying
```

Closes #34020 from dongjoon-hyun/SPARK-36759-2.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit adbea252db)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

docs/_config.yml

@@ -22,7 +22,7 @@ include:
 SPARK_VERSION: 3.2.1-SNAPSHOT
 SPARK_VERSION_SHORT: 3.2.1
 SCALA_BINARY_VERSION: "2.12"
-SCALA_VERSION: "2.12.14"
+SCALA_VERSION: "2.12.15"
 MESOS_VERSION: 1.0.0
 SPARK_ISSUE_TRACKER_URL: https://issues.apache.org/jira/browse/SPARK
 SPARK_GITHUB_URL: https://github.com/apache/spark

pom.xml

@@ -3360,7 +3360,7 @@
          SPARK-34774 Add this property to ensure change-scala-version.sh can replace the public `scala.version`
          property correctly. 
         -->
-        <scala.version>2.12.14</scala.version>
+        <scala.version>2.12.15</scala.version>
       </properties>
       <build>
         <pluginManagement>