Revert "Bump Go version to 1.23.6 (#30242)" (#30359)

This reverts commit acbbd4c58d.

server/public/model/client4.go

@@ -774,7 +774,7 @@ func (c *Client4) DoAPIRequestReader(ctx context.Context, method, url string, da
 		rq.Header.Set(HeaderAuth, c.AuthType+" "+c.AuthToken)
 	}
 
-	if len(c.HTTPHeader) > 0 {
+	if c.HTTPHeader != nil && len(c.HTTPHeader) > 0 {
 		for k, v := range c.HTTPHeader {
 			rq.Header.Set(k, v)
 		}