can i download an older os version on my old ipad so i can use the backup to activate my new one

No.

If the backup was created from a version lower than or equal to that on the new iPad, restore it as is.

If the backup was created from a version higher than that on the new iPad, set it up as a new device, update it, erase it, and set it up again.

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Rolling-back to an earlier version of iOS/iPadOS is not possible.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.

While rolling back simply isn’t possible, neither should it be necessary. A backup of an iOS/iPadOS device can be restored to a device that has the same or later version of iOS/iPadOS. Given that your new device will almost certainly have a more recent the equivalent OS installed, your existing backup should be fully usable.