Use Mono 6.6 and more recent versions of everything else

[HebaruSan]

Problem

Recent net-core builds have been failing.

Error: The assembly name is invalid.

Today my Ubuntu upgraded from Mono 6.6 to Mono 6.8, and suddenly I was affected locally by the same problem for all builds.

Cause

See cake-build/cake#2695, it looks like a check was added to Mono in mono/mono#16446 to reject full pathnames for assemblies, but unfortunately parts of Microsoft.CodeAnalysis (closed source?) appear to pass full pathnames, which breaks Cake. This will probably need to be fixed on the Mono side, but I don't yet have enough information for a useful report.

The net-core builds were set to mono: latest instead of a specific version like everything else, so when this (apparently) broken version shipped we started using it immediately.

Changes

To fix the error:

• Now our net-core builds use Mono 6.6 instead of latest

General upkeep:

• Now we build on bionic instead of xenial
• Versions of Mono older than 5.16 are dropped, 6.0 through 6.6 are added, and we'll ship the one from 6.6
• NuGet and Cake and Cake.SemVer versions are updated
• net-core version is updated from 2.1 to 3.1

[DasSkelett]

I guess you tested on Linux? I currently don't have access to my Linux machine, but on Windows it works fine for me after updating the VS build tools (had no NetCore 3.1 installed yet).

[HebaruSan]

OK, I managed to restore a Mono 6.6 install, and I'm able to compile and run CKAN.

[DasSkelett]

As I already wrote, works on W10, can't test on Linux as I'm on the road currently.

[HebaruSan]

I'll give @Olympic1 another few days to weigh in, in case I've missed something important about these versions.

[politas]

I'm still away from home. Will test on Manjaro when I get home.

[Olympic1]

Successfully build on Windows