Remove PROJ4 CMake name

[mwtoews]

PROJ 7.0 changed the CMake project name from PROJ4 to PROJ (#1885, #1910).

PROJ 9.1 made find_package(PROJ4) show a deprecation warning (#3094, #3165).

PROJ 10.0 should finally remove this support.

[rouault]

to solve OSGeo/gdal#5646, maybe we should offer a CMake variable to opt to disable exporting PROJ4, as something doable in PROJ 9.x series ?

[mwtoews]

Fine with me to bump this earlier in the 9.x series, but I also like to look further at the GDAL issue.

[dg0yt]

... target "proj" that is not in this export set, but in multiple other export sets: lib/cmake/proj/proj-targets.cmake, lib/cmake/proj/proj4-targets.cmake, lib/cmake/proj4/proj-targets.cmake, lib/cmake/proj4/proj4-targets.cmake

Instead of creating multiple exports of proj, it should be exported only once under the canonical name, and every alternative name should have been created as an interface library which depends on the canonical name.