Write script to find TODOs mentioning closed github issues

[mpguerra]

Motivation

There are a number of TODO comments in the code which refer to issues which have already been closed.

In order to get a full list of all of these comments and issues, we are going to try to write some scripting that finds TODOs that mention github issues that have been closed and update the list in #6281.

[mpguerra]

Hey team! Please add your planning poker estimate with Zenhub @arya2 @conradoplg @dconnolly @oxarbitrage @teor2345 @upbqdn

[conradoplg]

A reference: https://github.com/ZcashFoundation/developers/blob/main/zcash-issue-dag.py has code to check if an issue has been closed or not