[SPARK-32073][R] Drop R < 3.5 support

[HyukjinKwon]

What changes were proposed in this pull request?

Spark 3.0 accidentally dropped R < 3.5. It is built by R 3.6.3 which not support R < 3.5:

Error in readRDS(pfile) : cannot read workspace version 3 written by R 3.6.3; need R 3.5.0 or newer version.

In fact, with SPARK-31918, we will have to drop R < 3.5 entirely to support R 4.0.0. This is inevitable to release on CRAN because they require to make the tests pass with the latest R.

Why are the changes needed?

To show the supported versions correctly, and support R 4.0.0 to unblock the releases.

Does this PR introduce any user-facing change?

In fact, no because Spark 3.0.0 already does not work with R < 3.5.
Compared to Spark 2.4, yes. R < 3.5 would not work.

How was this patch tested?

Jenkins should test it out.

[HyukjinKwon]

Jenkins R's 3.4 IIRC. I will see the test results first, and let the appropriate person know and update the env.

[SparkQA]

Test build #124421 has finished for PR 28908 at commit d48ecf9.

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

[shivaram]

LGTM. Thanks @HyukjinKwon

[dongjoon-hyun]

Thanks, @HyukjinKwon . BTW, if we accidentally dropped it, how can we mention it in this case? This PR should target Apache Spark 3.0.1 with releasenotes label? (For now, it's just a normal JIRA issue.)
cc @gatorsmile and @srowen

[viirya]

As we accidentally dropped R < 3.5 in Spark 3.0.0, can we update the doc like https://spark.apache.org/docs/latest/index.html to make it less confusing?

[HyukjinKwon]

@viirya, it will be ported back to branch-3.0 and 3.0.1 will contains the doc fixes there. It should be fine with 3.0.1.

[HyukjinKwon]

Sure, I added releasenotes label in the JIRA.

[HyukjinKwon]

Oh, just as a note to my self, Jenkins R version seems to be 3.5.2:

using R version 3.5.2 (2018-12-20)

Nice.

[HyukjinKwon]

Merged to master, branch-3.0 and branch-2.4.

[shivaram]

Oh, just as a note to my self, Jenkins R version seems to be 3.5.2:

using R version 3.5.2 (2018-12-20)

Nice.

Hah - we should file a JIRA to upgrade it to 4.0 or at least 3.6.3?

[HyukjinKwon]

I think so .. I will at least target to test 4.0 in AppVeyor at #28909 for now