upgrade CI to R 4.0

[jangorecki]

closes #4406
.gitattributes partially address #3450 - it is needed so initial git checkout on windows uses proper newline (as is in the repository), not converting to windows because that invalidate fread tests of newline.

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changes in our source dockerfiles

upgraded from ubuntu 16.04 to ubuntu 18.04

• r-base-dev
• r-builder
• r-devel

upgraded from R 3.6 to R 4.0

• r-base-dev
• r-builder

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changes in windows runners

• private H2O windows CI runner machine has been replaced by public GitLab windows shared runners
• r-release upgraded to R 4.0
• r-devel now upgrades automatically

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CI status: https://gitlab.com/jangorecki/data.table/pipelines/143350906

test-dev-win has 2 failed tests

• one will be fixed by fixes R-devel foverlaps failures due to c.POSIXct change #4428
• second issue is related to tests of base R order, not data.table, discussed on email so far

test-dev-cran-lin has 1 failed tests

• will be fixed by fixes R-devel foverlaps failures due to c.POSIXct change #4428

[codecov]

Codecov Report

Merging #4431 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4431      +/-   ##
==========================================
- Coverage   99.61%   99.60%   -0.01%     
==========================================
  Files          72       72              
  Lines       13917    13917              
==========================================
- Hits        13863    13862       -1     
- Misses         54       55       +1     

Impacted Files	Coverage Δ
src/assign.c	99.84% <0.00%> (-0.16%)	⬇️

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