Remove PackageCloud CI templates, jobs, and variables

Summary

Removes all PackageCloud CI upload machinery from the pipeline. PackageCloud (packages.gitlab.com) has been superseded by Pulp for package distribution, and PACKAGECLOUD_ENABLED has been set to "false" (disabled) for some time. This MR makes that permanent by removing the dead code entirely.

Part of #9737.

Changes

• Remove 5 PackageCloud CI templates: .skip_unless_packagecloud_enabled, .packagecloud_staging_upload_template, .packagecloud_fips_staging_upload_template, .packagecloud_production_release_template, .packagecloud_fips_release_template
• Remove 31 concrete staging jobs extending those templates (e.g. Ubuntu-22.04-staging, Debian-12-staging, etc.)
• Remove 31 concrete release jobs extending those templates (e.g. Ubuntu-22.04-release, AlmaLinux-9-release, etc.)
• Remove PACKAGECLOUD_ENABLED: "false" from gitlab-ci-config/variables.yml
• Update .raspbian_release_template to extend .production_release_template directly (its previous parent .packagecloud_production_release_template was removed)

What is NOT changed

• .raspbian_release_template and its two concrete jobs (Raspberry-Pi-2-Bullseye-release, Raspberry-Pi-2-Bookworm-release) - intentionally kept
• All -pulp variant jobs - these are the replacement and are unaffected
• packagecloud.io/cinc-project gem source URLs - these are Chef/Cinc gem hosting, unrelated to package uploads