[backport#17304 2/18] Refactor: Declare LegacyScriptPubKeyMan methods as virtual

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/533d8b364f4e589aa1acb28cdea5e6e6e80d34dc

Depends on D7147

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7148