.gitmodules: use https:// instead of git://
Per https://github.blog/2021-09-01-improving-git-protocol-security-github/, support for unencrypted git://
protocol over port 9418 was dropped on 2022-03-15.
Per https://github.blog/2021-09-01-improving-git-protocol-security-github/, support for unencrypted git://
protocol over port 9418 was dropped on 2022-03-15.