Added support for .Net Standard 2.0

[kabilanvk]

LOG4NET-572: Add target for netstandard-2.0

[KamranShahid]

Needed log4net for atleast .net standard 2.0.
Problem in publishing app of .net core 3.0 where log4net 2.0,8 is referenced

[kabilanvk]

Needed log4net for atleast .net standard 2.0.
Problem in publishing app of .net core 3.0 where log4net 2.0,8 is referenced

My change is to get a new build for .net standard 2.0 but the current build system doesn't support it. Need a new build system setup.

[dpsenner]

I am afraid nant is no longer able to cope with this task. We would have to replace all what nant does with something else. This could be msbuild, paired with custom inline build tasks written with c# code. Feel free to go for it! Contributions welcome!

[kabilanvk]

@dpsenner
I don't know whether you got my emails about i've shown my interest in contributing to Log4Net.

I have set up a sample build for Log4Net.standard solution using azure devops. Let me know if its okay to proceed then we can migrate all builds to azure devops.
https://dev.azure.com/logging-log4net/Log4Net/_build/results?buildId=20&view=artifacts

[cremor]

Seems like #59 added .NET Standard 2.0 targets.

[fluffynuts]

Good day

It's been quite a while since this PR last saw activity. In an attempt to get pull requests under some semblance of
order, I'm having to make the uncomfortable decision to jettison pull requests which have been dormant for quite some
time. Perhaps arbitrarily, I'm choosing all pull requests which have not seen activity this year (2021), most of
which are marked as not building or have conflicts with the main branch.

This is not because the contributions aren't valuable - it's simply a matter of being the only person spending
some time on this and being a little overwhelmed. I'd rather get some traction on newer issues than continue
to try to understand, upgrade and work through PRs which don't build or which conflict with the main branch,
or which simply may have been solved in the mean time.

I encourage you to re-submit the PR against the current main branch if the issue is still significant. Your
contributions are appreciated. I apologise for any inconvenience caused and sincerely hope that you understand
the constraints involved which have brought me to the place of making this decision.