go: bump to 1.14

The oldest distros we support are Fedora 32 and RHEL 8.3. As both have
Go 1.14, we're safe to upgrade.

Also, I had to change prepare-source.sh because go fmt now refuses to run on
a project which has issues in go.mod, go.sum or modules.text. I think this
should be a harmless change.

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 1.13
+      - name: Set up Go 1.14
         uses: actions/setup-go@v1
         with:
-          go-version: 1.13
+          go-version: 1.14
         id: go
 
       - name: Check out code into the Go module directory