Remove Debian buster

Debian buster cannot build Rust 1.41.1 on armel.  It is also no longer
an operating system we officially support.  Remove support for it.

Signed-off-by: brian m. carlson <[email protected]>

.github/workflows/ci.yml

@@ -2,53 +2,6 @@ name: ci
 on: [push, pull_request]
 
 jobs:
-  build-buster:
-    name: Linux (Debian buster amd64)
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-ruby@v1
-    - run: make ci-buster
-    - uses: actions/upload-artifact@v2
-      with:
-        name: buster-amd64-deb
-        path: target/assets/muter_*.deb
-  build-buster-armel:
-    name: Linux (Debian buster armel)
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-ruby@v1
-    - run: |
-        echo '{"experimental": true}' | sudo tee /etc/docker/daemon.json
-        sudo systemctl restart docker.service
-        docker version -f '{{.Server.Experimental}}'
-    - uses: docker/setup-qemu-action@v1
-    - run: make ci-buster
-      env:
-        PLATFORM: linux/arm/v5
-    - uses: actions/upload-artifact@v2
-      with:
-        name: buster-armel-deb
-        path: target/assets/muter_*.deb
-  build-buster-arm64:
-    name: Linux (Debian buster arm64)
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-ruby@v1
-    - run: |
-        echo '{"experimental": true}' | sudo tee /etc/docker/daemon.json
-        sudo systemctl restart docker.service
-        docker version -f '{{.Server.Experimental}}'
-    - uses: docker/setup-qemu-action@v1
-    - run: make ci-buster
-      env:
-        PLATFORM: linux/arm64
-    - uses: actions/upload-artifact@v2
-      with:
-        name: buster-arm64-deb
-        path: target/assets/muter_*.deb
   build-bullseye:
     name: Linux (Debian bullseye amd64)
     runs-on: ubuntu-latest

Makefile

@@ -1,5 +1,5 @@
 # Test configuration.
-GROUPS := buster bullseye stable nightly oldest
+GROUPS := bullseye stable nightly oldest
 DOCKER_FILES := $(patsubst %,test/Dockerfile.%,$(GROUPS))
 DOCKER_STAMPS := $(patsubst %,test/Dockerfile.%.stamp,$(GROUPS))
 CI_TARGETS := $(patsubst %,ci-%,$(GROUPS))