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ARROW-18061: [CI][R] Reduce number of jobs on every commit (#14420)

* Force tests: `true` fully contains all tests in the `false` version, so false is redundant
* The CentOS 7 job previously was useful to catch gcc 4.8 issues, but that's no longer a problem, and the CentOS with devtoolset-8 variant is already tested nightly
* The Windows builds can be pruned if we test only on current release and development version, which only use UCRT. We test the older build nightly in the `r-binary-packages` workflow

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>