rockspec: use git+https:// for git repository URL
rockspec: use git+https:// for git repository URL

GitHub is going to disable unencrypted Git protocol, so `git://` URLs
will stop working soon (see [1]).

[1]: https://github.blog/2021-09-01-improving-git-protocol-security-github/

Part of https://github.com/tarantool/tarantool/issues/6587

tarantool/tarantool-avro,tarantool/tarantool-avro,tarantool/tarantool-avro,tarantool/tarantool-avro,tarantool/tarantool-avro,tarantool/tarantool-avro,tarantool/tarantool-avro