MSRV goes to R 3.6

[VisruthSK]

Remove pipes, lambdas, etc. so that the minimum R version is 3.6 instead of the unnecessarily strict 4.2 from before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
R/cite_usage.R	100.00% <100.00%> (ø)
R/collect.R	100.00% <100.00%> (ø)
R/scan_usage.R	100.00% <100.00%> (ø)
R/universe.R	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if b40bcb8 is merged into v0.3.0:

• ✔️Statistical_Rethinking_with_brms_ggplot2_and_the_tidyverse: 745ms -> 735ms [-3.66%, +1.11%]
• ✔️ggplot2: 467ms -> 468ms [-0.94%, +1.15%]
• ✔️loo: 462ms -> 468ms [-1.62%, +4.41%]
• ✔️loo_knitr: 437ms -> 432ms [-2.43%, +0.26%]
  Further explanation regarding interpretation and methodology can be found in the documentation.