remove v1.11 legacy docs from backport tool

Author: shuangela

Diff for: .backportrc.json

@@ -4,7 +4,7 @@
   "repoName": "docs-golang",
 
   // the branches available to backport to
-  "targetBranchChoices": ["master", "v2.0", "v1.17", "v1.16", "v1.15", "v1.14", "v1.13", "v1.12", "v1.11"],
+  "targetBranchChoices": ["master", "v2.0", "v1.17", "v1.16", "v1.15", "v1.14", "v1.13", "v1.12"],
 
   // Optional: automatically merge backport PR
   "autoMerge": true,