Use vendors when go generate (#7340)

* use vendors when go generate

* update docs about golang minimal requirement from 1.9 to 1.11

* fix build

.drone.yml

 
   - name: build-without-gcc
     pull: always
-    image: golang:1.10 # this step is kept as the lowest version of golang that we support
+    image: golang:1.11 # this step is kept as the lowest version of golang that we support
+    environment:
+      GO111MODULE: on
     commands:
-      - go build -o gitea_no_gcc # test if build succeeds without the sqlite tag
+      - go build -mod=vendor -o gitea_no_gcc # test if build succeeds without the sqlite tag
 
   - name: build
     pull: always

Makefile

 
 .PHONY: generate
 generate:
-	GO111MODULE=on $(GO) generate $(PACKAGES)
+	GO111MODULE=on $(GO) generate -mod=vendor $(PACKAGES)
 
 .PHONY: generate-swagger
 generate-swagger:

docs/content/doc/advanced/hacking-on-gitea.en-us.md

 on the executable path. If you don't add the go bin directory to the
 executable path you will have to manage this yourself.
 
-**Note 2**: Go version 1.9 or higher is required; however, it is important
+**Note 2**: Go version 1.11 or higher is required; however, it is important
 to note that our continuous integration will check that the formatting of the
 source code is not changed by `gofmt` using `make fmt-check`. Unfortunately,
 the results of `gofmt` can differ by the version of `go`. It is therefore

docs/content/doc/installation/from-source.en-us.md

 on the executable path. If you don't add the go bin directory to the
 executable path, you will have to manage this yourself.
 
-**Note 2**: Go version 1.9 or higher is required. However, it is recommended to
+**Note 2**: Go version 1.11 or higher is required. However, it is recommended to
 obtain the same version as our continuous integration, see the advice given in
 <a href='{{< relref "doc/advanced/hacking-on-gitea.en-us.md" >}}'>Hacking on
 Gitea</a>