Update wording for preferences > brave starts with > Dashboard

[bsclifton]

Now says "a new tab"

Fixes #6521

I created #6823 to address the core issue (so that users with newtab => blank can access the tabs).

Auditors: @bradleyrichter, @luixxiul

• 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).

Test Plan

1. Go to about:preferences
2. Ensure "Brave starts with" dropdown no longer has "Dashboard"; it should have "a new tab"