The problem is actually with your intel gpu.
[ 21266.188] (WW) modeset(0): Present-flip: queue async flip during flip on CRTC 0 failed: Invalid argument
Async flip was enabled in kernel 5.11 and later and doesn’t work properly on some older hardware, leading to the display output to freeze. So when you upgraded from Mint 20.3 (kernel 5.4) to 21 (kernel 5.15) you ran into this.
You mght want to use the liquorix ppa to get the latest kernel to check if this has been fixed meanwhile.
Another workaround would be downgrading the mint kernel to GA, if possible, which should be still 5.4 and stick to that.
https://wiki.ubuntu.com/Kernel/LTSEnablementStack
IIRC, another workaround is disabling PRIME sync
https://forums.developer.nvidia.com/t/ubuntu-18-04-3-blank-screen-at-startup-with-430-drivers-and-gtx-960/107501/2?u=generix
but this will result in tearing and disabling external outputs on the nvidia gpu.
Furthermore, you should check with the i915 issue tracker if there’s some work done.