Change OTLP/HTTP port from 4317 to 4318 (#1839) (#1970)

Related to open-telemetry/opentelemetry-specification#1816
Fixes open-telemetry/opentelemetry-specification#1835
Fixes open-telemetry/opentelemetry-specification#1920

Some historical context: we wanted to make grpc and http use the same port and we
had an open issue in the Collector to do so:
open-telemetry/opentelemetry-collector#1256

The conclusion is that there are technical hurdles that make it unfeasible:
open-telemetry/opentelemetry-collector#1256 (comment)

Because of that we need to keep grpc and http ports separate. This means we need to
change the spec to say that otlp/http uses port 4318. Once this PR is merged we
will also need to submit for port 4318 registration with IANA like we did previously
with port 4317:
open-telemetry/opentelemetry-specification#1148 (comment)

There was also a draft PR to merge the ports in the Collector but it was not completed:
open-telemetry/opentelemetry-collector#3765

Note that this change was initially submitted in PR open-telemetry/opentelemetry-specification#1839
and then reverted in PR open-telemetry/opentelemetry-specification#1847
because we hoped that the merging could be successfully done. We believe that we should
no longer pursue this and should consider the ports separate from now on.

Note 2: we consider this a spec bug fix (spec was impossible to implement), rather than a
functionality change, that's why we believe this is an allowed change.

specification/protocol/otlp.md

@@ -478,7 +478,7 @@ connections SHOULD be configurable.
 
 #### OTLP/HTTP Default Port
 
-The default network port for OTLP/HTTP is 4317.
+The default network port for OTLP/HTTP is 4318.
 
 ## Implementation Recommendations