Move static-nodes.json write location

[bkero]

This commit moves the static-nodes.json file from being written to
$DATADIR/geth/static-nodes.json to $DATADIR/static-nodes.json.

Testing from bkero and kjowiak confirmed that the file is actually read
from $DATADIR/

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[Slava]

lgtm

[ryanml]

master: 5cc25fd
0.24.x: cd37100
0.23.x: 01096cb