Upgrade jsonschema library to 5.2.0 (#349)

* make jsonschema loading work with current versions of the jsonschema library
   again and simplify the code while at it.
* Improve itest workflow for validation service, to make them more robust.
    * Use a github action to setup nodejs and use a version that is
       required by https://github.com/secvisogram/csaf-validator-service/.
    * Add nodejs16 installation to prepareUbuntuInstanceForITests.sh.
       (so it can be done once in manual settings)
    * Add refreshing of apt cache before apt install because sometimes
       the cached ubuntu image does not have the apt cache current.

---------

Co-authored-by: Bernhard Reiter <bernhard@intevation.de>

.github/workflows/itest.yml

@@ -11,11 +11,17 @@ jobs:
         with:
           go-version: 1.19.1
 
+      - name: Set up Node.js
+        uses: actions/setup-node@v3
+        with:
+          node-version: 16
+
       - name: Checkout
         uses: actions/checkout@v3
 
       - name: Execute the scripts
         run: |
+          sudo apt update
           sudo apt install -y make nginx fcgiwrap gnutls-bin
           cp -r $GITHUB_WORKSPACE ~
           cd ~