TT#124273 github actions: replace buster with bullseye

As of commit f607c7d and its raise of debhelper compat level to v13,
we no longer support Debian/buster, as builds are failing with:

| The following packages have unmet dependencies:
|  builddeps:. : Depends: debhelper-compat (= 13)
| E: Unable to correct problems, you have held broken packages.

Replace our github action for Debian/buster (current oldstable)
with Debian/bullseye (current stable).

Change-Id: Ia4289454bd807351e6bb9389ff50bffd1587f043

.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