bump minimum go version to 1.15

The oldest distributions that we now support are RHEL 8.4 and Fedora 33.
They both support go 1.15, let's bump.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

.github/workflows/tests.yml

@@ -16,10 +16,10 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
 
-      - name: Set up Go 1.14
+      - name: Set up Go 1.15
         uses: actions/setup-go@v1
         with:
-          go-version: 1.14
+          go-version: 1.15
         id: go
 
       - name: Check out code into the Go module directory