Bumped release stage to beta (#16102)

Author: lazebnyi

airbyte-config/init/src/main/resources/seed/source_definitions.yaml

@@ -928,7 +928,7 @@
  documentationUrl: https://docs.airbyte.io/integrations/sources/slack
  icon: slack.svg
  sourceType: api
- releaseStage: alpha
+ releaseStage: beta
 - name: Smartsheets
  sourceDefinitionId: 374ebc65-6636-4ea0-925c-7d35999a8ffc
  dockerRepository: airbyte/source-smartsheets