[ci] temporarily remove R3.6 CRAN CI build (#5049)
[ci] temporarily remove R3.6 CRAN CI build (#5049)


henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM