chore: remove stable release tag (#8885)

# Which Problems Are Solved

The current "stable" release tag was no longer maintained.

# How the Problems Are Solved

Remove the tag from the docs.

# Additional Changes

Update the docs to reflect that test run with Ubuntu 22.04 instead of
20.04.

# Additional Context

- relates to https://github.com/zitadel/zitadel/issues/8884

.github/workflows/release-channels.yml

@@ -1,47 +0,0 @@
-name: ZITADEL Release tags
-
-on:
-  push:
-    branches:
-      - "main"
-    paths:
-      - 'release-channels.yaml'
-  workflow_dispatch:
-
-permissions:
-  contents: write
-  packages: write
-
-jobs:
-  Build:
-    runs-on: ubuntu-20.04
-    env:
-      DOCKER_BUILDKIT: 1
-    steps:
-      - name: Source checkout
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
-      - name: get stable tag
-        run: echo STABLE_RELEASE=$(yq eval '.stable' release-channels.yaml) >> $GITHUB_ENV
-      - name: checkout stable tag
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
-          ref: ${{ env.STABLE_RELEASE }}
-      - name: GitHub Container Registry Login
-        uses: docker/login-action@v3
-        with:
-          registry: ghcr.io
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
-      - name: Google Artifact Registry Login
-        uses: docker/login-action@v3
-        with:
-          registry: europe-docker.pkg.dev
-          username: _json_key_base64
-          password: ${{ secrets.GCR_JSON_KEY_BASE64 }}
-      - name: copy release to stable
-        run: |    
-          skopeo --version
-          skopeo copy --all  docker://ghcr.io/zitadel/zitadel:$STABLE_RELEASE docker://ghcr.io/zitadel/zitadel:stable