Make ArgsManager.GetDataDirPath private and drop needless suffix

Summary:
This is a backport of core#21850 [4/6]
https://github.com/bitcoin/bitcoin/pull/21850/commits/13bd8bb0536f008118b1de921654925ed9ce1da7

Depends on D11954

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11955