[TINKERPOP-2085]: Remove Mono dependency from gremlin-dotnet build/release

[Cole-Greer]

https://issues.apache.org/jira/browse/TINKERPOP-2085

Removes Mono from the gremlin-dotnet packaging/deploy pipeline, replacing mono nuget.exe pack/push with native dotnet pack/dotnet nuget push. Mono was only needed to run the Windows-only nuget.exe on Linux; the dotnet CLI has supported equivalent pack/push functionality since SDK 2.1+, so no functionality is
lost.

One behavioral change for release managers: the modern dotnet nuget push has no equivalent to the old nuget.exe setApiKey, which stored the key encrypted (via Windows DPAPI) in NuGet.Config — that encryption isn't available on Linux/macOS. Deploys now require an explicit -Dnuget.apiKey=[key] Maven property,
with a fail-fast check if it's omitted. (NuGet/Home#6437)

Changes:

• Replace mono nuget.exe pack → dotnet pack for Gremlin.Net.Template (adds / to its .csproj)
• Replace mono nuget.exe push → dotnet nuget push, gated behind a new -Dnuget.apiKey property with a fail-fast Ant check
• Remove now-dead nugetVersion/nugetExe properties and the ant-contrib antrun dependency
• Remove Mono APT repo, GPG key import, and mono-devel from docker/Dockerfile
• Update development-environment.asciidoc and release.asciidoc for the new pack/push/API-key workflow

I haven't yet tested this against a deployment to a live environment, my intention is to pull this forward to master, and try it out during the upcoming deployment of 4.0.0-beta.3. I will make any adjustments as necessary during that process.

VOTE +1

[xiazcy]

VOTE +1

[Cole-Greer]

Merging as CTR