meson: Raise minimum glib version to 2.56.0
g_file_load_bytes(), used in tests/pixbuf-gif.c, was introduced in 2.56.0 according to https://developer.gnome.org/gio/stable/GFile.html#g-file-load-bytes
Found when building on a system with glib 2.52.0:
../tests/pixbuf-gif.c: In function ‘run_gif_test’:
../tests/pixbuf-gif.c:93:17: error: implicit declaration of function ‘g_file_load_bytes’; did you mean ‘g_file_load_contents’? [-Werror=implicit-function-declaration]
93 | input_bytes = g_file_load_bytes (input_file, NULL, NULL, &error);
| ^~~~~~~~~~~~~~~~~
| g_file_load_contents