chore(emqx_lwm2m): pin lwm2m-coap v2.0.0

v2.0.0 has emqx_http_lib added as a rebar dependency
v1.* is for emqx v4.3.*

apps/emqx_lwm2m/rebar.config

@@ -1,5 +1,7 @@
 {deps,
- [{lwm2m_coap, {git, "https://github.com/emqx/lwm2m-coap", {tag, "v1.1.5"}}}
+  %% lwm2m-coap v.1.* is for emqx v4.3.*
+  %% lwm2m-coap v.2.* is for emqx v5.*
+ [{lwm2m_coap, {git, "https://github.com/emqx/lwm2m-coap", {tag, "v2.0.0"}}}
  ]}.
 
 {profiles,