Fetch CF ranges concurrently and prevent writes

Use tokio::join to fetch IPv4 and IPv6 Cloudflare ranges in parallel.
When range fetch fails, avoid performing updates that could write
Cloudflare addresses by clearing detected/filtered IP lists and emitting
warnings. Add unit tests to validate parsing and boundary checks for the
current Cloudflare ranges. Bump crate version to 2.0.6.
Fetch Cloudflare ranges concurrently; avoid writes

Skip updates (clear detected IPs) if Cloudflare ranges can't be
retrieved to avoid writing Cloudflare anycast addresses.
Default REJECT_CLOUDFLARE_IPS=true, update README, add comprehensive
CF-range tests, and bump crate version
Fetch CF ranges concurrently and avoid updates

Enable rejecting Cloudflare IPs by default and skip any updates
if the published ranges cannot be fetched to avoid writing Cloudflare
anycast addresses. Fetch IPv4 and IPv6 ranges concurrently, add
parsing/matching tests, and update README and version.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cloudflare-ddns"
-version = "2.0.5"
+version = "2.0.6"
 edition = "2021"
 description = "Access your home network remotely via a custom domain name without a static IP"
 license = "GPL-3.0"