heads-up: PROJ_LIB is now deprecated (use PROJ_DATA instead)

[jmckenna]

• PROJ_LIB will now be deprecated, and PROJ_DATA should be used instead
• see PROJ changes at Introduce PROJ_DATA environment variable to deprecate PROJ_LIB OSGeo/PROJ#3253
• I believe some builds of libosmium point to PROJ_LIB
• likely both PROJ_LIB and PROJ_DATA should be checked, for now (similar to how MapServer will handle this, see Apply special processings of PROJ_LIB to PROJ_DATA as well MapServer/MapServer#6573 )
• the big change will be part of the upcoming PROJ 9.1 release, but it's good for all projects to be ready for this