Use new methods in the io and os packages instead of ioutil ones

ioutil is deprecated in Go 1.16 and SFTPGo is an application, not
a library, we have no reason to keep compatibility with old Go
versions.

Go 1.16 fix some cifs related issues too.

.github/workflows/development.yml

@@ -15,9 +15,9 @@ jobs:
         os: [ubuntu-latest, macos-latest]
         upload-coverage: [true]
         include:
-          - go: 1.15
-            os: ubuntu-latest
-            upload-coverage: false
+          #- go: 1.15
+          #  os: ubuntu-latest
+          #  upload-coverage: false
           - go: 1.16
             os: windows-latest
             upload-coverage: false
@@ -268,7 +268,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - name: Set up Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: 1.16
       - name: Run golangci-lint
         uses: golangci/golangci-lint-action@v2
         with:
           version: latest
+          skip-go-installation: true