[SPARK-27467][FOLLOW-UP][BUILD] Upgrade Maven to 3.6.1 in AppVeyor and Doc

[wangyum]

What changes were proposed in this pull request?

Update the docs/building-spark.md. Otherwise:

mvn package -DskipTests=true
...
[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (enforce-versions) @ spark-parent_2.12 ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireMavenVersion failed with message:
Detected Maven Version: 3.6.0 is not in the allowed range 3.6.1.
...
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M2:enforce (enforce-versions) on project spark-parent_2.12: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [Help 1]
[ERROR]
...

How was this patch tested?

Just test https://archive.apache.org/dist/maven/maven-3/3.6.1/binaries/apache-maven-3.6.1-bin.zip is avilable.

[SparkQA]

Test build #104960 has finished for PR 24477 at commit 76a734b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

+1, LGTM. Merged to master.
cc @HyukjinKwon

[HyukjinKwon]

Let me revert this as of #24480. Not sure about the root cause, hm.