Hi himanshu49,
sorry to say it that hard, but as far as I know there is no way to downgrade your photos library back to photos 8.
You should never ever switch hard drives between different MacOS versions, bad things can happen.
Here one option to recover your photos in version 8.
Buy a new hard drive, connect this to your older MacBook
Format this hard drive exactly as described!
Move your Photos library to save space on your Mac - Apple Support
Create a new system photos library on the new hard drive
Create additional photo libraries in Photos on Mac - Apple Support
Setup iCloud on both MacBooks
Set up iCloud on your Mac - Apple Support
Get an iCloud plan that will fit all your photos
iCloud+ plans and pricing - Apple Support
Then wait a few days until all your photos moved from your current hard drive to iCloud and then down to your new hard drive. Time will depend on the amount of photos and videos that you have.
After you have verified that all photo are download to the new hard drive in photos 8 you could cancel / downgrade your iCloud plan.
A second option would be to export all photos and import them in the new library.
But that is complicated and you would loos your folder / album structure.
Hope it will help
Regards
Ralf