[SPARK-30453][BUILD][R] Update AppVeyor R version to 3.6.2

[HyukjinKwon]

What changes were proposed in this pull request?

R version 3.6.2 (Dark and Stormy Night) was released on 2019-12-12. This PR targets to upgrade R installation for AppVeyor CI environment.

Why are the changes needed?

To test the latest R versions before the release, and see if there are any regressions.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

AppVeyor will test.

[HyukjinKwon]

Seems it's working fine:

R version 3.6.2 (2019-12-12) -- "Dark and Stormy Night"
Copyright (C) 2019 The R Foundation for Statistical Computing
Platform: i386-w64-mingw32/i386 (32-bit)
...

[dongjoon-hyun]

+1, LGTM. Merged to master. Thank you, @HyukjinKwon .
To reviewers: AppVeyor test is already finished and Jenkins result is irrelevant.

Downloading R-win.exe (86,419,784 bytes)...100%
R is now available on drive C:
R version 3.6.2 (2019-12-12)
Platform: i386-w64-mingw32/i386 (32-bit)
Running under: Windows Server 2012 R2 x64 (build 9600)

[SparkQA]

Test build #116273 has finished for PR 27124 at commit f1a1368.

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