replaced ubuntu 16.04 with 18.04 and 20.04

[dragosmg]

closes #858

summary: ubuntu-16.04 is deprecated which results in pipeline failures. this PR solves that by moving to ubuntu 18.04 and 20.04. it also makes use of {pak} to speed up dependency installations.

[dragosmg]

this now uses {pak} for package and dependency installations.
good to go when / if checks passed.
the requirement to pass on ubuntu-16.04 & current R version needs to be removed - I think only @jimhester might be able to do this.

[jimhester]

Thanks!