build: add img.promote target

[jbw976]

Description of your changes

This PR addresses the publish-artifacts failure in https://github.com/crossplane/conformance/actions/runs/17748244007/job/50438010383:

make[2]: *** No rule to make target 'img.promote'.  Stop.

We simply follow the pattern used in the last c/c release that still used make: https://github.com/crossplane/crossplane/blob/release-1.16/cluster/images/crossplane/Makefile#L34-L37

I have:

• Read and followed conformance's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

I have tested the build, publish, and now also promote path that ci.yml uses by first updating REGISTRY_ORGS ?= docker.io/jbw976 to push to my personal account, then running:

# build and publish, as before in https://github.com/crossplane/conformance/pull/32
❯ make -j2 build.all
❯ make -j2 publish BRANCH_NAME=main

# now actually exercise promote
❯ make -j2 promote BRANCH_NAME=main CHANNEL=master
09:19:15 [ .. ] docker promote docker.io/jbw976/conformance:v2.0.0-cf.1.rc.0.2.gb12f7b0-dirty to docker.io/jbw976/conformance:master
[+] Building 1.8s (1/1) FINISHED
 => [internal] pushing docker.io/jbw976/conformance:master                                                                                                                                                     1.8s
09:19:18 [ OK ] docker promote docker.io/jbw976/conformance:v2.0.0-cf.1.rc.0.2.gb12f7b0-dirty to docker.io/jbw976/conformance:master

This results in a successful publish/promote of index.docker.io/jbw976/conformance:master:
https://hub.docker.com/repository/docker/jbw976/conformance/tags/master/sha256-9a47abad459c4d88705d45be654741a23accf84da08dbeeda6aa3ff37f00032e