Fix links to GDK `main` (from `master`)

What does this MR do?

It seems that we recently renamed the branch master -> main, but didn't update all the links.

I grepped in our repo and that's what I found.

Maybe we should have a redirect for such cases? 🤔

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because it is documentation change.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

Opened the updated links, checked that they are resolving correctly (unlike the ones we had before, with the master).

Security

N/A