chore: move tracing-opentelemetry to its own repo (tokio-rs#2523)

This PR removes tracing-opentelemetry to a dedicated repo located at
https://github.com/tokio-rs/tracing-opentelemetry. (Note that at time of
writing this PR, the new repo has not be made public). We're moving
tracing-opentelemetry to a dedicated repository for the following
reasons:
1. opentelemetry's MSRV is higher than that of `tracing`'s.
2. more importantly, the main `tracing` repo is getting a bit unweildy
   and it feels unreasonable to maintain backports for crates that
   integrate with the larger tracing ecosystem.

(https://github.com/tokio-rs/tracing-opentelemetry does not have the
examples present in this repo; this will occur in a PR that will be
linked from _this_ PR.)

.github/workflows/CI.yml

@@ -227,7 +227,6 @@ jobs:
         - tracing-serde
         - tracing-subscriber
         - tracing-tower
-        - tracing-opentelemetry
         - tracing
         toolchain:
         - 1.49.0

examples/Cargo.toml

@@ -24,6 +24,7 @@ tracing-serde = { path = "../tracing-serde" }
 tracing-appender = { path = "../tracing-appender" }
 tracing-journald = { path = "../tracing-journald" }
 tracing-appender = { path = "../tracing-appender", version = "0.2.0" }
+tracing-journald = { path = "../tracing-journald" }
 
 # serde example
 serde_json = "1.0.82"