Update Go versions

Author: rtpt-alexanderneumann

.github/workflows/linters.yml

@@ -9,7 +9,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v1
         with:
-          go-version: 1.14.x
+          go-version: 1.15.x
         id: go
 
       - name: Install golang-ci

.github/workflows/tests.yml

@@ -6,10 +6,8 @@ jobs:
     strategy:
       matrix:
         go-version:
-          - 1.11.x
-          - 1.12.x
-          - 1.13.x
           - 1.14.x
+          - 1.15.x
     runs-on: ubuntu-latest
 
     env: