vulkan: Demote g_critical() to g_warning()

For VK_DEBUG_REPORT_WARNING_BIT_EXT we should always have used
g_warning().

For VK_DEBUG_REPORT_ERROR_BIT_EXT g_critical() is technically the right
choice, but Mesa has been using this flag for normal warnings, so until
that gets fixed, we don't want to throw criticals.

Related: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31292
Fixes: !7020