drop support for deprecated App-ID auth backend (#61) (#78)

App-ID is deprecated since Vault 0.6 and was removed in 1.12.
Our compatibility methods are deprecated since Connector 0.4. It's time
to drop it for good.

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
         vault: [ '1.2.0', '1.11.12', '1.17.6' ]
         include:
           - jdk: 21
-            vault: '1.11.12'
+            vault: '1.17.6'
             analysis: true
     steps:
       - name: Checkout