FindProtobuf: Fix new version number scheme

Revise the changes from commit fc7dcc6a24 (FindProtobuf: Support new
version number scheme, 2023-06-19, v3.27.0-rc3~2^2).  Changing the
content of `Protobuf_VERSION` to exclude the major version of the library
might break code as it can't be used to detect breaking changes in the
library any more.

However, protoc v22 and up don't print the major version any more, so we
need to compare the against a truncated version number, too.