Bump Confluent.Kafka from 2.6.1 to 2.8.0 in /tracer/dependabot

Bumps [Confluent.Kafka](https://github.com/confluentinc/confluent-kafka-dotnet) from 2.6.1 to 2.8.0.
- [Release notes](https://github.com/confluentinc/confluent-kafka-dotnet/releases)
- [Changelog](https://github.com/confluentinc/confluent-kafka-dotnet/blob/master/CHANGELOG.md)
- [Commits](confluentinc/confluent-kafka-dotnet@v2.6.1...v2.8.0)

---
updated-dependencies:
- dependency-name: Confluent.Kafka
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

tracer/dependabot/Datadog.Dependabot.Integrations.csproj

@@ -54,7 +54,7 @@
     <!-- Integration: Confluent.Kafka -->
     <!--    Assembly: Confluent.Kafka -->
     <!-- Latest package https://www.nuget.org/packages/Confluent.Kafka/2.6.1 -->
-    <PackageReference Include="Confluent.Kafka" Version="2.6.1" />
+    <PackageReference Include="Confluent.Kafka" Version="2.8.0" />
 
     <!-- Integration: Couchbase.NetClient -->
     <!--    Assembly: Couchbase.NetClient -->