TT#124273 GitHub actions: upgrade from buster to bullseye

In commit a4ee01d27f we switched to debhelper compat level 13.
debhelper v13 isn't available in (plain) Debian buster,
and therefore the Debian pipeline for buster fails with:

| The following packages have unmet dependencies:
|  builddeps:. : Depends: debhelper-compat (= 13)

Since we updated packaging for bullseye, also adapt the GitHub
actions / workflow accordingly.

Change-Id: I411a6ab16ee48c5c5fc1ca43c58c016df83e1ff7

.github/actions/debpkg-bullseye/Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:buster
+FROM debian:bullseye
 
 COPY entrypoint.sh /entrypoint.sh
 

.github/actions/debpkg-bullseye/action.yml

@@ -0,0 +1,6 @@
+name: "Build Docker image based on Debian/bullseye"
+description: "Build Docker image based on Debian/bullseye"
+
+runs:
+  using: 'docker'
+  image: 'Dockerfile'

.github/actions/debpkg-buster/action.yml

@@ -1,6 +0,0 @@
-name: "Build Docker image based on Debian/buster"
-description: "Build Docker image based on Debian/buster"
-
-runs:
-  using: 'docker'
-  image: 'Dockerfile'

.github/workflows/debpkg.yml

@@ -7,16 +7,16 @@ on:
     - cron: '0 8 * * *'
 
 jobs:
-  build-deb-buster:
+  build-deb-bullseye:
     runs-on: ubuntu-latest
-    name: Debian pipeline for buster
+    name: Debian pipeline for bullseye
 
     steps:
     - name: Checkout source
       uses: actions/checkout@v2
 
     - name: Execute Docker debpkg action
-      uses: ./.github/actions/debpkg-buster
+      uses: ./.github/actions/debpkg-bullseye
 
     - name: Store Debian package artifacts
       uses: actions/upload-artifact@v2