Replace deprecated occ app:check-code with xml linter

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/workflows/app-code-check.yml

@@ -1,32 +0,0 @@
-name: PHP AppCode Check
-
-on:
-  pull_request:
-  push:
-    branches:
-      - master
-
-jobs:
-  unit-tests:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        php-versions: ['7.4']
-        nextcloud-versions: ['stable20']
-    name: AppCode check
-    steps:
-      - name: Set up php${{ matrix.php-versions }}
-        uses: shivammathur/setup-php@master
-        with:
-          php-version: ${{ matrix.php-versions }}
-          extension-csv: ctype,curl,dom,gd,iconv,intl,json,mbstring,openssl,posix,sqlite,xml,zip
-          coverage: xdebug
-      - name: Checkout Nextcloud
-        run: git clone https://github.com/nextcloud/server.git --recursive --depth 1 -b ${{ matrix.nextcloud-versions }} nextcloud
-      - name: Checkout
-        uses: actions/checkout@master
-        with:
-          path: nextcloud/apps/calendar
-      - name: App Code check
-        working-directory: nextcloud
-        run: php occ app:check-code calendar