chore(source-db2): Convert to new gradle build flow (do not merge) (#62915)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: yue.li@airbyte.io <yue.li@airbyte.io>

airbyte-integrations/connectors/source-db2/build.gradle

@@ -1,5 +1,7 @@
 plugins {
     id 'airbyte-java-connector'
+    id "io.airbyte.gradle.docker"
+    id 'airbyte-connector-docker-convention'
 }
 
 airbyteJavaConnector {

airbyte-integrations/connectors/source-db2/metadata.yaml

@@ -13,7 +13,7 @@ data:
     - suite: integrationTests
   connectorType: source
   definitionId: 447e0381-3780-4b46-bb62-00a4e3c8b8e2
-  dockerImageTag: 0.3.0
+  dockerImageTag: 0.3.1
   dockerRepository: airbyte/source-db2
   documentationUrl: https://docs.airbyte.com/integrations/sources/db2
   githubIssueLabel: source-db2