Fix configuration of ghaction-import-gpg (#18)

Motivation:

We did update the ghaction-import-gpg action but did miss to update the
configuration as well

Modifications:

Fix configuration of action

Result:

Be able to release again

.github/workflows/ci-release.yml

@@ -40,7 +40,7 @@ jobs:
         id: import_gpg
         uses: crazy-max/ghaction-import-gpg@v6
         with:
-          gpg-private-key: ${{ secrets.GPG_PRIVATE_KEY }}
+          gpg_private_key: ${{ secrets.GPG_PRIVATE_KEY }}
           passphrase: ${{ secrets.GPG_PASSPHRASE }}
 
       - name: Setup git configuration