Move .gitmodules from git protocol due to deprecation

[rgdoliveira]

This is related to:

• https://github.blog/2021-09-01-improving-git-protocol-security-github/
• https://blog.readthedocs.com/github-git-protocol-deprecation/

[rgdoliveira]

@samskivert could you please review it?

[samskivert]

Thanks!