refactor!: Remove deprecated features

Both `with-coap-message` and `example-server_coaphandler`
no longer have to be activated as features. Since they're now buildable
on stable rust, they're regular dependencies instead.

Cargo.toml

@@ -29,12 +29,6 @@ coap-handler-implementations = "0.5.0"
 default = ["std"]
 std = ["lru_time_cache"]
 
-# The dependencies behind these features hasve become non-optional when they
-# became buildable on stable Rust. The features are deprecated to use, and can
-# be removed in a breaking release.
-with-coap-message = []
-example-server_coaphandler = []
-
 # UDP feature enables additional optimizations for CoAP over UDP.
 udp = []