replace dead links (fixes #3333)

Important: Do not tick a checkbox if you haven’t performed its action.

[x] Verify the changes are on a new branch other than master – it should say "compare: branch-name".
[x] Start the pull request title with lowercase letters, e.g., add githubusername.md.
[x] Add issue number to the end of the pull request title when applicable, e.g., update mi-faq.md (fixes #3264).

[x] Go to "Commits" tab, make sure the commit username is clickable and linked to your GitHub account correctly.
[x] Review the "Files changed" tab to ensure there are no unnecessary files or changes included in the pull request.
[x] Verify that the raw.githack preview link is included in the description.
- [x] Preview the MDwiki rendered changes using the raw.githack link. Ensure it displays as expected without any errors.
[x] Drop a link to this pull request in our discord channel.

Replaced the dead links in both mi-github-and-markdown.md and mi-github-issues.md.

fixes #3333

open-learning-exchange / open-learning-exchange.github.io