GitHub CI: update list of container images

Remove explicit versions, and instead use :latest and other aliases. Try
to get a latest stable release and a development release for several
well-known distributions. Drop CentOS as it is no longer supported.

.github/workflows/test.yml

@@ -24,7 +24,7 @@ jobs:
  - mingw
 
  container:
- image: debian:bullseye
+ image: debian:stable
  options: --privileged
 
  steps:
@@ -59,7 +59,7 @@ jobs:
  runs-on: ubuntu-22.04
  timeout-minutes: 20
  container:
- image: debian:bullseye-slim
+ image: debian:stable-slim
  env:
  CI: 1
 
@@ -197,17 +197,16 @@ jobs:
  fail-fast: false
  matrix:
  os:
- - alpine
+ - alpine:latest
  - alpine:edge
- - centos:7 # aka RHEL 7
- - almalinux:8 # aka RHEL 8
- - almalinux:9 # aka RHEL 9
- - fedora
- - debian:buster
- - debian:bullseye
+ - almalinux:latest
+ - fedora:latest
+ - fedora:rawhide
+ - debian:oldstable
+ - debian:stable
  - debian:testing
- - ubuntu # current LTS
- - ubuntu:rolling # latest
+ - ubuntu:latest
+ - ubuntu:rolling
  container:
  image: ${{ matrix.os }}
  options: --privileged