fix(ci): remove unsupported s390x/ppc64le platforms from docker-buildx

PLATFORMS included linux/s390x and linux/ppc64le, but the base image
quay.io/zncdatadev/go-devel:1.25.8-kubedoop0.0.0-dev does not provide
manifests for these architectures, causing Release Image to fail with
'no match for platform in manifest: not found'.

Align with other operator projects that only build linux/arm64,linux/amd64.

Author: whg517

Makefile

@@ -125,7 +125,7 @@ docker-push: ## Push docker image with the manager.
 # - have enabled BuildKit. More info: https://docs.docker.com/develop/develop-images/build_enhancements/
 # - be able to push the image to your registry (i.e. if you do not set a valid value via IMG=<myregistry/image:<tag>> then the export will fail)
 # To adequately provide solutions that are compatible with multiple platforms, you should consider using this option.
-PLATFORMS ?= linux/arm64,linux/amd64,linux/s390x,linux/ppc64le
+PLATFORMS ?= linux/arm64,linux/amd64
 BUILDX_METADATA_FILE ?= docker-digests.json	# The file to store the digests of the images built by buildx
 .PHONY: docker-buildx
 docker-buildx: ## Build and push docker image for the manager for cross-platform support