Support using label names when changing issue labels (#30943) (#30958)

Backport #30943 by @Zettat123

Resolve #30917

Make the APIs for adding labels and replacing labels support both label
IDs and label names so the
[`actions/labeler`](https://github.com/actions/labeler) action can work
in Gitea.

<img width="600px"
src="https://github.com/go-gitea/gitea/assets/15528715/7835c771-f637-4c57-9ce5-e4fbf56fa0d3"
/>

Co-authored-by: Zettat123 <zettat123@gmail.com>

templates/swagger/v1_json.tmpl

@@ -21754,12 +21754,9 @@
       "type": "object",
       "properties": {
         "labels": {
-          "description": "list of label IDs",
+          "description": "Labels can be a list of integers representing label IDs\nor a list of strings representing label names",
           "type": "array",
-          "items": {
-            "type": "integer",
-            "format": "int64"
-          },
+          "items": {},
           "x-go-name": "Labels"
         }
       },