Previously, we were conditionally trying to set `min_header_version` to
the lowest possible value, and relying on a "legacy" Google interface to
determine the file descriptor's syntax version as part of that
determination.

Instead, simply bump the minimum version to 1003000 (1.3.0). This
release was almost 7 years ago. In practice protobuf-c users should not
be shipping pre-compiled .pb-c.c/.pb-c.h files, anyway.