Yet another round of EOL additions and URL fixes
This round we're:
- Removing GNOME 3.6 ISO URL, as it's been constantly failing in our tests;
- Adjusting CentOS 6.10 URLs after it reached its EOL;
- Adjusting CentOS 8 EOL;
- Updating CentOS 8 URLs and ISO information due to the 8.3.2011 release;
- Adjusting Scientific Linux 6.* releases after it reached its EOL;
- Adding Fedora 31 and Silverblue 31 EOL information (and adjust its URLs);
- Updating Debian 9 URLs;
- Updating Debian 10 URLs;
- Adjusting Ubuntu 19.10 tree URLs;