gh-actions: use ubuntu 20.04

[GregorDeCillia]

ubuntu 18.04 is no longer supported on gh-actions since 2023-04-01. this pr changes the gh-actions config to use ubuntu 20.04 instead.

[GregorDeCillia]

backtrace for the failing tests

devtools::install_deps()
  * VIM: Can't install dependency car
  * car: Can't install dependency pbkrtest (>= 0.4-4)
  * pbkrtest: Needs R >= 4.1.0

{pbkrtest} was re-published on CRAN on 2023-01-19 and the required R version was then bumped from 3.5.0 to 4.1.0. oldrel-2, oldrel-3 and oldrel-4 currently point to the R versions 4.0.5, 3.6.3 and 3.5.3. The last gh-actions run for simPop was before this release on 2022-11-18

Possible fixes

• remove oldrel-2, oldrel-3 and oldrel-4 from the build config
• make sure the {rbkrtest} version is between 0.4-4 and 0.5.1 for thise images

I'm not sure if this issue would actually occur if someone with R 4.0.5 would install this package from CRAN. See https://github.com/r-lib/pak/issues/290 for more info on how dependencies are resolved.

What do you think @bernhard-da ?

[GregorDeCillia]

Even though the failing tests have nothing to do with changing the ubuntu images, I suggest we resolve those issues in this branch before merging.

[GregorDeCillia]

Two more points

1. The folks form tidyverse and r-lib only use ubuntu-latest in their workflows
2. It might be possible to move VIM to Suggests because it is only used in one place via VIM::hotdeck()

https://github.com/statistikat/simPop/blob/208a25edef63254f925e328861b210116fd1f0bc/R/simContinuous.R#L866-L869

[GregorDeCillia]

related: oldrel-2, oldrel-3 and oldrel-4 were removed in 5871417eb695281fb5c714dd53a18aa627a03bf4 and this resulted in successfull ci tests.