Remove Dependabot monitoring for retired Go imgs

Remove monitoring for:

- `/mirror/1.19`
- `/mirror/1.20`

Neither of those Go release series are receiving further
updates from the Go team.

refs: GH-1404

.github/dependabot.yml

@@ -333,52 +333,6 @@ updates:
           - ">= 1.23"
           - "< 1.22"
 
-  - package-ecosystem: docker
-    directory: "/mirror/1.19"
-    open-pull-requests-limit: 10
-    target-branch: "master"
-    schedule:
-      interval: "daily"
-      time: "02:00"
-      timezone: "America/Chicago"
-    assignees:
-      - "atc0005"
-    labels:
-      - "dependencies"
-      - "CI"
-    allow:
-      - dependency-type: "all"
-    commit-message:
-      prefix: "Mirror Build Image"
-    ignore:
-      - dependency-name: "amd64/golang"
-        versions:
-          - ">= 1.20"
-          - "< 1.19"
-
-  - package-ecosystem: docker
-    directory: "/mirror/1.20"
-    open-pull-requests-limit: 10
-    target-branch: "master"
-    schedule:
-      interval: "daily"
-      time: "02:00"
-      timezone: "America/Chicago"
-    assignees:
-      - "atc0005"
-    labels:
-      - "dependencies"
-      - "CI"
-    allow:
-      - dependency-type: "all"
-    commit-message:
-      prefix: "Mirror Build Image"
-    ignore:
-      - dependency-name: "amd64/golang"
-        versions:
-          - ">= 1.21"
-          - "< 1.20"
-
   - package-ecosystem: docker
     directory: "/mirror/1.21"
     open-pull-requests-limit: 10