Use go 1.15 at minimum (#3224)

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
             couchdb-version: 2.3.1
             lint: true
           # More exotic version
-          - go-version: 1.13.x
+          - go-version: 1.15.x
             couchdb-version: 3.1.1
     steps:
       - name: Install CouchDB

docs/INSTALL.md

@@ -39,7 +39,7 @@ executable bit (`chmod +x cozy-stack`) and put it in your `$PATH`.
 
 #### Using `go`
 
-[Install go](https://golang.org/doc/install), version >= 1.13. With `go`
+[Install go](https://golang.org/doc/install), version >= 1.15. With `go`
 installed and configured, you can run the following command:
 
 ```

go.mod

 module github.com/cozy/cozy-stack
 
-go 1.13
+go 1.15
 
 require (
 	github.com/Masterminds/semver/v3 v3.1.1