The 5.15 Kernel is a bit older yes. But it is an LTS Kernel, so it is still supported. When Your System works good, you don't need to install a newer Kernel. So, the Question is: Work your System without Problems?
If You want a newer Kernel you could simply install Zorin 17. Then You have Kernel 6.2. If that ist not enough you can manually install a newer Kernel with Ubuntu Mainline. For this is here a Tutorial: How to upgrade Zorin 16.2 to a newer kernel
But You do this on Your own Risk. So ask yourself if You really NEED a newer Kernel.