GHA migration from ubuntu-16.04 (deprecated)

[dragosmg]

ubuntu-16.04 environment is deprecated and will be removed on September 20, 2021. Migrate to ubuntu-18.04 or ubuntu-latest.

this is already affecting lintr - see PR #857 R CMD check failing on ubuntu-16.04.

for more details see actions/runner-images#3287

You will need to migrate ubuntu-16.04 workflows to ubuntu-18.04 or ubuntu-latest which will run on Ubuntu 20.04 LTS.