Issue #480: Fix gh action

[nikosbosse]

Fixes #480

As discussed in #480, Github action checks for R 3.5 are currently failing. This is because some packages that scoringutils imports or suggests depend on R >= 3.6.

This PR

• makes the dependency on R 3.6 explicit in DESCRIPTION
• updates gh action to use R 3.6 instead of R 3.5
• replaces |> with %>% in some tests that failed because of the new pipe operator

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (develop@4df4576). Click here to learn what that means.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #483   +/-   ##
==========================================
  Coverage           ?   80.21%           
==========================================
  Files              ?       20           
  Lines              ?     1713           
  Branches           ?        0           
==========================================
  Hits               ?     1374           
  Misses             ?      339           
  Partials           ?        0           

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