Upgrade default R version to something newer

[sunu]

Opening this as a separate issue, as suggested by @manics in #1567 (comment), to continue the discussion.

As @wildintellect mentioned in that thread:

In the R world major.minor.patch is a actually a big deal on the minor part. Minor is about once per year and often has really important changes.

The current pinned version is now close to 2 years old.

Proposed change

Upgrade the default R version to something newer in

repo2docker/repo2docker/buildpacks/r.py

Line 78 in 808ae6b

r_version = version_map["4.4"]

@manics suggested applying the same policy we use for Python:

For Python we say

Starting in 2023, the default version of Python used when Python version is unspecified will be updated more often. Python itself releases a new version every year now, and repo2docker will follow, with the default Python version generally trailing the latest stable version of Python itself by 1-2 versions.

If you choose not to specify a Python version, your repository is guaranteed to stop working, eventually. We strongly recommend specifying a Python version (in environment.yml, runtime.txt, Pipfile, etc.)

https://repo2docker.readthedocs.io/en/latest/howto/languages/#supported-versions-of-python

I think we could do the same for R? If a repo is going to break due to a change in the unpinned R version it's better for it to happen sooner when there's a chance the author is still around.

Alternative options

Don't upgrade for now and keep the same default.

Who would use this feature?

R users who haven't pinned a version.

How much effort will adding it take?

Bumping the default version is minimal. Updating the docs to match the Python approach, if we decide to go that route, is a bit more.