cmake/FindProj.cmake still looks for proj_api.h ?

[landryb]

https://github.com/MapServer/MapServer/blob/main/cmake/FindProj.cmake#L9 still looks for proj_api.h to detect the presence of proj, this will fail with proj 8 which removed the header.

simply looking for proj.h seems to be enough for 7.6.2 to properly detect proj 8 presence.

-- Found PROJ: /usr/local/lib/libproj.so.16.0  
-- Found Proj 8.0

i guess this was forgotten in #5766..

--- cmake/FindProj.cmake.orig
+++ cmake/FindProj.cmake
@@ -6,7 +6,7 @@
 #    PROJ_LIBRARY
 
 
-FIND_PATH(PROJ_INCLUDE_DIR proj_api.h)
+FIND_PATH(PROJ_INCLUDE_DIR proj.h)
 
 FIND_LIBRARY(PROJ_LIBRARY NAMES proj proj_i)