Skip hdKeypath of 'm'

Summary:

Previously the seed was stored with keypath 'm' so we need to skip this
as well when determining inactive seeds.

Partial backport of core PR19054:
https://github.com/bitcoin/bitcoin/pull/19054/commits/3a03a11e8c696e2164b8bb221a4a35a7c3ac4d6d

This unbreaks opening the wallets (since D9196) that were created before
D4175.

There is a second commit with an upgrade test which requires some work
to be ported.

Test Plan:

ninja all check-all

Checked I can now open my wallet.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9211