apt: update salt repo URL to packages.broadcom.com

[DasSkelett]

As we all know, Salt is now owned by Broadcom, and they moved the packages to their repository.
https://saltproject.io/blog/salt-project-package-repo-migration-and-guidance/

This adjusts the repository and signing key URLs.
In theory they are now the same for all architectures and (deb-based) diistros, however our guardians running oldoldstable Raspbian (10/buster) need armhf/armv7l/32-bit builds, while Broadcom only offers arm64, so upgrades are still not possible.

N: Skipping acquire of configured file 'main/binary-armhf/Packages' as repository 'https://packages.broadcom.com/artifactory/saltproject-deb stable InRelease' doesn't support architecture 'armhf'

[herbetom]

Just as a comment:

There hasn't been an update to salt and tbh the salt repo doesn't look like there will be one shortly. And if there is one at some point i'm not overly confident that there won't be some fuckup.
So far i've just removed the old source in other salt deployments. But maybe at least pin the current version or something like it? Adding a source requires trust and i wouldn't be too confident with the current saltstack situation that trust is justified.