[ANNOUNCE] New Kubernetes and Go versions in test and build

[Wilfred Spiegelenburg <wi...@apache.org>]

As part of a recent number of changes we have moved to later versions of
Kubernetes and Go.

The first change was building with a new version of go (1.15). There have
been no changes to the go.mod file which means that the code syntax is
still based on go 1.12. Newer language constructs are not supported as yet.
If we need new features from the language we need to update the mod file to
show that.
The minimum go version is no longer hardcoded in the Makefile but based on
the go.mod file.

The e2e tests now test against new releases of Kubernetes. K8s 1.14 and
1.15 have been removed and replaced with the latest 1.17 and 1.18 releases.
For v0.10 we will update the versions we test against and know that work in
the documentation.

Wilfred