[CI][R] Reduce number of jobs on every commit

[asfimport]

• 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

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #14420

_{Note: This issue was originally created as ARROW-18061. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 14420
#14420