Move wallet upgrading to its own function

Summary: Backport of Core PR15761 [1/6] : https://github.com/bitcoin/bitcoin/pull/15761/commits/9c16b1735f8e530ce68d678e9ca0eceb2ceb3520

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7922