Fix crash in scenes with more than 8 shadowed directional lights

[Namey5]

Looks like there was an off-by-one bounds check when culling directional lights - this was only causing a crash when all lights were shadowed however, as the culling data for shadows uses a fixed-size array.

Fixes #73854

[akien-mga]

This seems consistent with the Compatibility renderer, and the only occurrence:

$ rg ">.*MAX_DIRECTIONAL_LIGHTS"
servers/rendering/renderer_scene_cull.cpp
3286:                   if (directional_lights.size() > RendererSceneRender::MAX_DIRECTIONAL_LIGHTS) {

drivers/gles3/rasterizer_scene_gles3.cpp
1644:                           if (r_directional_light_count >= RendererSceneRender::MAX_DIRECTIONAL_LIGHTS || light_storage->light_directional_get_sky_mode(base) == RS::LIGHT_DIRECTIONAL_SKY_MODE_SKY_ONLY) {

[Repiteo]

Thanks!