CMake: Replace custom FindSqlite3 with FindSQLite3 built-in

[mwtoews]

This PR replaces the custom FindSqlite3 module with the FindSQLite3 built-in.

Upon further inspection, the variables SQLITE3_INCLUDE_DIR and SQLITE3_LIBRARY cannot be easily mapped to modern variables, so these variables are removed. Users are encouraged to use CMAKE_PREFIX_PATH instead, which is added to the install docs.

Closes #4004

[rouault]

SQLite3_INCLUDE_DIR (https://github.com/Kitware/CMake/blob/fa4a499238502dead01c482a168eb52b6599dc22/Modules/FindSQLite3.cmake#L37) and SQLite3_LIBRARY (https://github.com/Kitware/CMake/blob/fa4a499238502dead01c482a168eb52b6599dc22/Modules/FindSQLite3.cmake#L41) can still be used

[mwtoews]

Ah, thanks for pointer. I can map these, and provide a deprecation message. I'd like to still remove these from the docs, as these are advanced variables that aren't even in the CMake docs.

[mwtoews]

A small deprecation notice is added to docs here:

although I might be misusing the deprecated:: directive, so the formatting looks a bit off. It is normally indented under an option.

For consistency, we may want to also undocument CURL_ and TIFF_ variables for LIBRARY and INCLUDE_DIR too, as these should ideally be handled by CMAKE_PREFIX_PATH. I'm also fine to put back SQLITE3_INCLUDE_DIR and SQLITE3_LIBRARY docs with their deprecated:: too, if that's preferred.

[dg0yt]

FTR the target variables are (meant to be) cache variables. But it probably works as is.