Cannot use free resolver IDs for ControlD DoH

### What is the issue?

Using the string `p2` for a ControlD resolver like https://controld.com/free-dns?freeResolverType=blockMalwareAndAds&helpPane=dns does not work as the text box requires a longer string.

### Steps to reproduce

1. Go to https://login.tailscale.com/admin/dns
2. Click Add nameserver -> Control D
3. Enter `p2`

Observe that the OK button in dialog is disabled.

### Are there any recent changes that introduced the issue?

_No response_

### OS

_No response_

### OS version

_No response_

### Tailscale version

_No response_

### Other software

_No response_

### Bug report

_No response_