fix(specs): drop singer from ingestion specs [skip-bc] (#4281)

Author: shortcuts

specs/ingestion/common/schemas/source.yml

@@ -400,7 +400,7 @@ DockerRegistry:
 
 DockerImageType:
   type: string
-  enum: [singer, custom, airbyte]
+  enum: [custom, airbyte]
   description: Image type.
 
 SourceInput:

specs/ingestion/common/schemas/task.yml

@@ -461,7 +461,7 @@ StreamingInput:
     - mapping
 
 DockerStreamsInput:
-  description: The selected streams of a singer or airbyte connector.
+  description: The selected streams of an airbyte connector.
   type: object
   properties:
     streams:

specs/ingestion/paths/sources/discover.yml

@@ -4,7 +4,7 @@ post:
   summary: Trigger a stream-listing request
   description: |
     Triggers a stream-listing request for a source.
-    Triggering stream-listing requests only works with sources with `type: docker` and `imageType: singer`.
+    Triggering stream-listing requests only works with sources with `type: docker` and `imageType: airbyte`.
   operationId: triggerDockerSourceDiscover
   x-acl:
     - addObject