How would I go about modifying a Default Classic Workflow, such that tickets are automatically set to Status: Closed after a specific number of days?
You don't! Workflows don't have timers.
What most of us do is have a filter for "status = resolved and updated < -2w" and we set up a Jelly service that says "run filter and push each resulting issue through the workflow".
See https://confluence.atlassian.com/display/JIRA/Jelly+Escalation
use jelly escalation:
https://confluence.atlassian.com/display/JIRA044/Jelly+Escalation
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