Update the moved Git repos for developers

What does this MR do?

Update 5 Git repos moved on github.com for local development environments:

• https://github.com/documentcloud/underscore => https://github.com/jashkenas/underscore
• https://github.com/twitter/flight => https://github.com/flightjs/flight
• https://github.com/mattermost/platform => https://github.com/mattermost/mattermost-server
• https://github.com/gaearon/redux => https://github.com/reduxjs/redux
• https://github.com/DrBoolean/mostly-adequate-guide => https://github.com/MostlyAdequate/mostly-adequate-guide

What are the relevant issue numbers?

Closes #52978 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [n/a] Tests added for this feature/bug
• Conforms to the code review guidelines
• [n/a] Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n/a] Conforms to the database guides