Make R 4.1 on Windows a new "special" in examples/check-full.yaml

[jennybc]

Capturing a recent slack discussion.

We should add R 4.1 + windows-latest to the standard matrix in examples/check-full.yaml.

The argument is similar to that for keeping R 3.6 + windows-latest around: making sure to test with a previous toolchain, Rtools35.

With the release of R 4.2, now it's important to test on R 4.1 + windows-latest to retain coverage of RTools40 and a native encoding that's not UTF-8.

For practical reasons, this means we will probably drop R 3.6 + windows-latest from our standard matrix, since we're already flirting with various limits. Specific packages could choose to retain R 3.6 + windows-latest in the matrix, if it has proven to be helpful in recent memory.

We support R release + 4 previous versions on Windows, macOS, and Linux. But we don't test that complete matrix, rather we test one complete row and one complete column in the table below PLUS we add any special high-risk combinations (e.g. R 4.1 + windows-latest).

           lin win mac
devel      +    -   -
release    +    +   +
oldrel-1   +    -   -
oldrel-2   +    -   -
oldrel-3   +    -   -
oldrel-4   +    -   -

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