COMP: Remove the retired cloudflare-ipfs.com ExternalData mirror

`CMake/ITKExternalData.cmake` still lists `cloudflare-ipfs.com` as an ExternalData mirror. Cloudflare retired that gateway and the hostname no longer resolves, so every fetch that reaches this entry spends a DNS failure before falling through to the next mirror.

Evidence

On `upstream/main`, `CMake/ITKExternalData.cmake` (mirror list, ~lines 62-63):

```cmake
# Cloudflare gateway
"https://cloudflare-ipfs.com/ipfs/%(hash)"
```

`host cloudflare-ipfs.com` returns no address — the name is retired (NXDOMAIN).

The other IPFS gateways in the list (`ipfs.io`, `ipfs.filebase.io`,
`gateway.pinata.cloud`, `dweb.link`) are unaffected by this issue.

Proposed work

Delete the two lines. No replacement is needed — the remaining gateways already
provide redundancy. If a Cloudflare gateway is still wanted, the current
hostname would have to be confirmed first rather than assumed.

Open CMake/ITKExternalData.cmake around the mirror list at lines 62-63 and confirm the retired Cloudflare gateway entry. Remove the two lines, then review the diff to ensure the other listed IPFS gateways remain unchanged; done means no cloudflare-ipfs.com entry remains.

Written by the indexing model from the issue text.