[1.18 ]Bump to go 1.15 when upstream does

[cjellick]

For 1.17 and 1.18 we will need to update the go version we use to build when upstream does. Can track this issue:
kubernetes/release#1517

From: kubernetes/kubernetes#94037

Usually we don't backport Golang minor updates to previous branches, but because Kubernetes 1.19 will be built on go1.15 and go1.14 is being skipped by the Kubernetes community, we need to update to go1.15 on previous branches as well to remain on a supported Golang version for the Kubernetes 1.18 and 1.17 support cycles.

I've put this in the 1.18.9 milestone, but we just need to keep an eye on when and if they get it done upstream for that release. If it doesn't then we shouldn't move either

[cjellick]

just for cross-reference:
#2230
#2235

[davidnuzik]

Unless I'm mistaken it looks like this wasn't finished upstream. I've bumped this to the next patch release slated for November.

[davidnuzik]

Now being tracked via kubernetes/release#1651
Looks like this is in progress now upstream based on a thread today in #sig-release in kubernetes slack.

[davidnuzik]

We should bump to 1.15.5 or newer.

[davidnuzik]

Closing. Not needed in 1.18 branch. Since upstream is not bumping the go version, we shall not as well.