fix: Remove unnecessary makevars.ucrt

[albersonmiranda]

Removes makevars.ucrt templates and updates tests and snaps. Closes #332 .

[JosiahParry]

Thank you @albersonmiranda ! The changes to libR-sys actually require that the minimum supported version of R is 4.2.

I think we should set the minimum supported version of R to 4.2. see #375

[JosiahParry]

@albersonmiranda can you add a note to the changelog? I think we can skip the lintr failure right now and add exclusions for the standalone files using the .lintr file https://lintr.r-lib.org/articles/lintr.html at a later point

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.88%. Comparing base (cbd93f6) to head (3f5efc6).
Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
R/source.R	76.41% <100.00%> (ø)
R/use_extendr.R	98.94% <ø> (-0.04%)	⬇️