Fix linking with Protobuf 23

Google introduced abseil as a new dependency of Protobuf, but CMake's
FindProtobuf hasn't been updated yet to link with abseil. Thus, CMake is
now advised to use the config provided by Protobuf instead of its own.
In case no config can be found, we fall back to the old behavior.

The Protobuf version constraint is removed since it only applies with
`P4C_USE_PREINSTALLED_PROTOBUF=ON` anyway, in which case we'll want to
use whatever preinstalled version is available.

Furthermore, the variables `PROTOBUF_LIBRARY` and
`PROTOBUF_PROTOC_EXECUTABLE` are set correctly so executing `protoc` and
linking with Protobuf doesn't fail.

Fix p4lang#4055
See also protocolbuffers/protobuf#12292

cmake/Protobuf.cmake

@@ -11,10 +11,15 @@ macro(p4c_obtain_protobuf)
       set(SAVED_CMAKE_FIND_LIBRARY_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES})
       set(CMAKE_FIND_LIBRARY_SUFFIXES .a)
     endif()
-    find_package(Protobuf 3.0.0 REQUIRED)
+    find_package(Protobuf CONFIG)
+    if(NOT Protobuf_FOUND)
+        find_package(Protobuf REQUIRED)
+    endif()
     if(ENABLE_PROTOBUF_STATIC)
       set(CMAKE_FIND_LIBRARY_SUFFIXES ${SAVED_CMAKE_FIND_LIBRARY_SUFFIXES})
     endif()
+    set(PROTOBUF_LIBRARY "protobuf::libprotobuf")
+    set(PROTOBUF_PROTOC_EXECUTABLE "protobuf::protoc")
   else()
     # Google introduced another breaking change with protobuf 22.x by adding abseil as a new
     # dependency. https://protobuf.dev/news/2022-08-03/#abseil-dep We do not want abseil, so we stay