[backport] Trivial: fix references to share/rpcuser (now share/rpcauth)

Summary:
c64cc1bd94 scripted-diff: Fix references to share/rpcuser (now share/rpcauth) (Mason Simon)

Pull request description:

Commit 3fdb297 renamed share/rpcuser to share/rpcauth but left references to the old path in code; this commit fixes the old references.

```
-BEGIN VERIFY SCRIPT-
git grep --files-with-matches 'share/rpcuser' src/*.cpp | xargs sed -i -E 's:share/rpcuser:share/rpcauth:g'
git grep --files-with-matches 'share/rpcuser' test/functional/*.py | xargs sed -i -E 's:share/rpcuser:share/rpcauth:g'
-END VERIFY SCRIPT-
```

Tree-SHA512: 0ae5c746cfbaf38683e636382a9d518bbce123d308b1c8d47c1b089cc2e64dd6a7098cdfb6606ed5a9abc867d948a14022c3f3065f64076086ded429a7fa3cc7

This is a backport of Core PR13664

Test Plan:

ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5707