Added shared credential for nelnet

[Tschrock]

nelnet.com and secure.nelnet.net serve the same site, including login and account pages, and you can log into your account on either domain. Note, the bare domain nelnet.net redirects to nelnet.com.

https://nelnet.com/welcome https://secure.nelnet.net/welcome

Overall Checklist

• [X] I agree to the project's Developer Certificate of Origin
• [X] The top-level JSON objects are sorted alphabetically
• [X] There are no open pull requests for the same update

for shared-credentials.json

• [X] There's evidence the domains are currently related (SSL certificates, DNS entries, valid links between sites, legal documents etc.)
• [X] If using shared, the new group serves login pages on each of the included domains, and those login pages accept accounts from the others. (For example, we wouldn't use a shared association from google.co.il to google.com, because google.co.il redirects to accounts.google.com for sign in.)
• [N/A] If using from and to, the new group, the from domain(s) redirect to the to domain to log in.

[Sameesunkaria]

Hi @Tschrock

Can you run tools/convert-shared-credential-to-legacy-format.rb to update quirks/websites-with-shared-credential-backends.json which is a generated file?