OTP25 compatibility

[mkschreder]

Got following warnings:

src/coap_dtls_socket.erl:47:10: Warning: ssl:ssl_accept/1 is removed; use ssl_handshake/1,2,3 instead

src/core_link.erl:52:5: Warning: http_uri:encode/1 is deprecated and will be removed in OTP 25; use uri_string functions instead
src/core_link.erl:54:5: Warning: http_uri:encode/1 is deprecated and will be removed in OTP 25; use uri_string functions instead

src/coap_responder.erl:30:10: Warning: pg2:get_members/1 is removed; this module was removed in OTP 24. Use 'pg' instead
src/coap_responder.erl:205:13: Warning: pg2:create/1 is removed; this module was removed in OTP 24. Use 'pg' instead
src/coap_responder.erl:206:18: Warning: pg2:join/2 is removed; this module was removed in OTP 24. Use 'pg' instead
src/coap_responder.erl:230:10: Warning: pg2:leave/2 is removed; this module was removed in OTP 24. Use 'pg' instead
src/coap_responder.erl:232:10: Warning: pg2:get_members/1 is removed; this module was removed in OTP 24. Use 'pg' instead
src/coap_responder.erl:233:15: Warning: pg2:delete/1 is removed; this module was removed in OTP 24. Use 'pg' instead

src/coap_client.erl:105:9: Warning: http_uri:parse/2 is deprecated and will be removed in OTP 25; use uri_string functions instead
src/coap_client.erl:126:20: Warning: http_uri:decode/1 is deprecated and will be removed in OTP 25; use uri_string functions instead