chore: migrate from Upbound to Crossplane build infrastructure

[gergely-szabo-sap]

This PR migrates the project from using Upbound's build infrastructure to the upstream Crossplane build system.

Changes

• Build submodule: Updated .gitmodules to use crossplane/build instead of upbound/build
• Multi-platform builds: Modified GitHub Actions workflows to build linux_amd64 and linux_arm64 images separately using make build PLATFORM=... instead of relying on docker buildx
• Package publishing: Switched from make publish to make xpkg.build and make xpkg.release.publish for crossplane package distribution
• Image building: Migrated from image.mk to imagelight.mk for simpler image builds
• Dockerfile consolidation:
  • Removed separate Dockerfiles for controller and configuration packages
  • Added new provider-cloudfoundry Dockerfile using distroless base image
• Project naming: Updated PROJECT_NAME and PROJECT_REPO to use crossplane-provider-{name} naming convention
• E2E tests: Simplified test configuration by removing separate controller image handling
• Documentation: Added local kind build and e2e test instructions to DEVELOPER.md

Motivation

Align with Crossplane's upstream build tools and eliminate dependency on Upbound-specific infrastructure.

[maximilianbraun]

👋 if we can make ARM possible in parallel, that would be awesome. Not sure how feasible that is.

[sdischer-sap]

Great work. Put some comments.

[gergely-szabo-sap]

👋 if we can make ARM possible in parallel, that would be awesome. Not sure how feasible that is.

Very good suggestion. Let's see first, whether the sequential approach works. Then I'd go for the parallel building.

[sdischer-sap]

Looks like something is failing in the builds. Let me know when I should do another review.

[schlingemann]

Looks good. Let's give it a try and create a RC