Override default build_root_image to use Go 1.23: release-4.15

[danchild]

This is an extension of of work in #274

The openstack-test suites use go version 1.20 which accepts language versions such as 1.20 in the go.mod. Something like 1.23.0 is rejected outright. Starting in go version 1.21, the patch version becomes mandatory when running go mod, and since OTE requires 1.23, the following error is returned when building the binary (CI defaults to using 1.20 for the build for release-4.15):

cd tests-extension && GOTOOLCHAIN=auto GOSUMDB=sum.golang.org go build -o /go/src/github.com/openshift/openstack-test/bin/openstack-test-tests-ext ./cmd/extension
go: errors parsing go.mod:
/go/src/github.com/openshift/openstack-test/tests-extension/go.mod:3: invalid go version '1.23.0': must match format 1.23

We need to keep openshift-tests around while testing OTE. So as a workaround, it's easier and more consistent to continue using the most recent OTE versions and use a build container that uses go 1.23. rhel-9-release-golang-1.23-openshift-4.15 does not exist however rhel-9-release-golang-1.23-openshift-4.16 does (it's earliest supported tag using Go 1.23).

After we migrate fully to OTE, this workaround will no longer be needed and therefore will be dropped from the repository.

[openshift-ci]

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[danchild]

/assign @mandre

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[mandre]

Let's see if this fixes our issue. I suspect test platform automation might have something to say when the image version doesn't match the branch, we'll see.
In this case I believe it's acceptable since this is just for building a test suite, and it doesn't end up in the product.

/lgtm
/approve

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