Update documentation regarding 18.04 + 20.04 supported/recommended

[consideRatio]

It seems like we support running TLJH on both 18.04 and 20.04 currently, and I think its reasonable to support the two latest LTS releases. Should we aim for that?

I figure there is docs to update with regards to this, and this issue represents that.

[manics]

Do you think we should have a formal support policy, e.g. the two most recent Ubuntu LTS, so people know in advance that 18.04 will be dropped when 22.04 comes out? Our should the choice be more ad-hoc?

[consideRatio]

I like the idea of makings sure there is always two LTS versions supported. From a maintenance perspective, it seems like Ubuntu release schedule is to support their LTS versions for exactly five years, which means they support 2-3 LTS releases depending on if its a odd/even year.

Practically though, I think supporting three LTS releases at any given time forces us to have logic compatible with three releases, which is more logic than I think is worth maintaining. Perhaps there no additional complexity, perhaps there are - i don't know. But anyhow, I think for this project, I suggest we go for declaring that we always aim to support the latest two LTS releases, but not the third latest LTS release - which means, that when we add support for 22.04, we allow ourselves to drop support for 18.04 - even though it would be maintained by canonical for <=1 additional year out of its 5 year life span.

[iameskild]

@manics @consideRatio I just deployed TLJH on Ubuntu 20.04 LTS (on AWS) without any issues. I'd be happy to update the docs to reflect the support for the previous n versions of Ubuntu (I agree that supporting more than 2 past version might be overkill).

Given that there are multiple references to Ubuntu versions throughout the docs, perhaps we can simply use Sphinx variable substitution and have one central place that needs updating as new versions of Ubuntu are released (and tested).

Keep on eye out for a PR by the end of the week :)