feat: Support api version 2 for nuget datasource (#3148)

lib/datasource/nuget/index.js

test/_fixtures/nuget/indexV3.json

test/_fixtures/nuget/latestV2.json

test/_fixtures/nuget/latestV2.xml

test/_fixtures/nuget/latestV2_withoutProjectUrl.xml

test/_fixtures/nuget/nunit.json

test/_fixtures/nuget/nunitV2.json

test/_fixtures/nuget/nunitV2.xml

test/_fixtures/nuget/nunitV3.json

test/_fixtures/nuget/nunitV3_withoutProjectUrl.json

test/_fixtures/nuget/nunitv3_nuget-org.xml

test/_fixtures/nuget/sample.nuspec

test/datasource/__snapshots__/nuget.spec.js.snap

test/datasource/nuget.spec.js

website/docs/nuget.md