CI/build/publishing updates

[nblumhardt]

• GitHub issue templates (redirecting usage questions to Stack Overflow)
• CI with GitHub Actions
• Update targets to include only supported platforms (net4.6.2+)
• CSPROJ updates for icon and license
• Update dependencies
• Mark version 4.0.0
• breaking Rename namespace Serilog.Sinks.ApplicationInsights.Sinks.ApplicationInsights.TelemetryConverters to Serilog.Sinks.ApplicationInsights.TelemetryConverters

If this builds, I'll add NUGET_API_KEY to the repository secrets and attempt to publish a -dev build.

See also #184

[nblumhardt]

Note that the fixing of namespaces will cause a flow-on change to be required in JSON configuration. Editing the PR notes now to mark this as a breaking change in v4.