netlink: drop support for Go 1.11 and below at Go 1.15 release

[mdlayher]

Go 1.12 was released in February 2019, and enabled us to integrate with the runtime network poller for deadline and SyscallConn support.

With Go 1.15 on the horizon, I'd like to officially drop support for Go 1.11 and below when I tag v1.2.0. This means that I will incorporate the few minor changes since v1.1.0 in a v1.1.1, but that version will be the last to support Go 1.11 and below.

I understand some users are stuck on LTS versions of Go shipped with their operating systems, but they will still be able to use v1.1.1 indefinitely until they can upgrade. The Go project policy is to only support the latest two versions of Go, which as of 1.15 would be 1.14 and 1.15. I think it's time to make a clean break and remove a lot of build tag shims.

[mdlayher]

I'm a little surprised nobody has mentioned anything about this yet. I take it this means that it's a good idea!

[mdlayher]

v1.1.1 has been tagged. Now I'll start removing support for Go 1.11.