Add tracing-opentelemetry to the OTel crate Dependabot group (#931)

Since its version is closely tied to that of the other `opentelemetry*`
crates, so it should be grouped/updated in the same Dependabot PR
as them.

xref #930

Author: edmorley

.github/dependabot.yml

@@ -12,6 +12,7 @@ updates:
       opentelemetry:
         patterns:
           - "opentelemetry*"
+          - "tracing-opentelemetry"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: