ci: Bump GH action runners to ubuntu-20.04

[morgsmccauley]

ubuntu-18.04 will be deprecated soon, they have already started introducing brown-outs which are causing our PR checks to fail.

This PR bumps the GH Action runner-images to ubuntu-20.04, this is the same version the lake nodes are running in GCP, so shouldn't have any impact, but I'll do some tests to verify. cross docker version shouldn't need to be updated as it's only the GH runners which are affected.

[khorolets]

I guess it's worth to be upgraded to 1.31.1 already, would you mind?

[morgsmccauley]

This PR wasn't meant to include this commit, I forgot to point it to #62. I'll merge this PR just with the CI fixes and create a new PR for 1.31.1.

[morgsmccauley]

@khorolets The required check needs to be updated

- test (linux-x86_64, ubuntu-18.04, stable, x86_64-unknown-linux-gnu)
+ test (linux-x86_64, ubuntu-20.04, stable, x86_64-unknown-linux-gnu)

[khorolets]

I wonder whether we expected that "linux"-something job to be completed?

[morgsmccauley]

I wonder whether we expected that "linux"-something job to be completed?

@khorolets That job will never run, it's the old job which is specified in the repo settings marking it as a required check. Matrix jobs specify the variables in the title, so since we've upgraded to 20.04, it's considered a new job, while the repo still expects the 18.04 job to run.

This is why I've suggested to change the required check here: #63 (comment)