🐛Default to EdgeIpVersion of "auto"

**Describe the bug**
By default, tunnels can't be created on IPv6-only systems since `EdgeIpVersion` defaults to `4` instead of `auto`. This has been discussed in #842 #954 #1019 but those issues have been closed without a change in the default. While it's possible to override the systemd file, that file needs to exist first so using `cloudflared service install` as given in the Cloudflare Dashboard does not result in a working tunnel

**To Reproduce**
Steps to reproduce the behavior:
1. Configure a remote tunnel in the Cloudflare Zero Trust Dashboard
2. Run the `cloudflare service install ...` command given by the dashboard on an IPv6-only device
3. See the command stall
4. Run `systemctl status cloudflared` and see that it's trying to use IPv4 even when there are no IPv4 addresses assigned to any interface (other than the `127.0.0.1` on `lo`)

If it's an issue with Cloudflare Tunnel:
* Tunnel ID : (did not get to registering/creating/connecting)
* cloudflared config: N/A (remote-managed tunnel)

**Expected behavior**
IPv6 should work out of the box (with no systemd overrides needed)

**Environment and versions**
- OS: Debian 13 (Trixie)
- Architecture: x64
- Version: 2025.8.0

**Logs and errors**
N/A

**Additional context**
* IPv6 support is something Cloudflare actively advertises, such as in this blog post (which, while it doesn't use tunnels in the example, still advertises being IPv6 ready): https://blog.cloudflare.com/amazon-2bn-ipv4-tax-how-avoid-paying/
* Not having IPv6 support by default is not well-documented unless you're looking for it, which means that, in addition to the commands in the Dashboard not being correct/complete, it requires spending time searching the docs and GitHub issues to figure out why it's happening
* Needing to override the systemd unit file adds a non-trivial amount of friction to using IPv6 and is not beginner-friendly