Replace libprotobuf with upb for dynamic decode

[Gold856]

libprotobuf is a very annoying dependency to deal with, and with the switch to nanopb for generated C++ code, libprotobuf is only used for dynamic decode in the GUI apps. libprotobuf has been swapped out with upb, a much smaller C-based library that supports reflection and can therefore do dynamic decode. This means we can remove the libprotobuf dependency and stop dealing with build issues because of it. Closes #6610 and closes #7250.

[Gold856]

Confirmed to be working on Windows:

Workflows need to be rerun, I can rerun them myself once the dependent PR is merged and/or we decide if this goes into main or 2027.

[PeterJohnson]

We're trying to minimize changes for 2026. Let's target this to 2027.

[Gold856]

I'll rebase this when #7983 gets merged into main and main gets merged into 2027.

[PeterJohnson]

I merged main to 2027 but there's some fixes I need to make so it fully builds again.

[Gold856]

I will note that protobuf@3 on Homebrew will be deactivated on July 1st, and while we could switch main to protobuf@29, which I confirmed would work, I would prefer that this gets shipped to main so we don't have to deal with it at all. I doubt that many users actually depend on libprotobuf directly, and I don't really think we considered it to be a public interface?