ci: Use Debian and Ubuntu release stage tags

Rather than use the release codename tags, use the release stage tags.
This way the configuration (theoretically) doesn't need to be updated
when new Debian and Ubuntu releases are made.

For Debian stable is used instead of buster and a testing (bullseye)
build is added. For Ubuntu, latest is used instead of focal for the
current LTS and rolling is used instead of groovy for the latest
release. This actually changes the Ubuntu build from groovy to hirsute.

.github/workflows/tests.yml

@@ -35,8 +35,13 @@ jobs:
 
       matrix:
         include:
-          - name: Debian Buster (10) with sign-ed25519
-            image: debian:buster-slim
+          # Debian builds. Currently stable and testing are tested.
+          # Other options would be stable-backports, oldstable,
+          # oldstable-backports and unstable.
+          #
+          # https://hub.docker.com/_/debian
+          - name: Debian Stable with sign-ed25519
+            image: debian:stable-slim
             pre-checkout-setup: |
               apt-get update
               apt-get install -y git
@@ -45,8 +50,8 @@ jobs:
             configure-options: >-
               --with-ed25519-libsodium
 
-          - name: Debian Buster (10) with curl, sign-ed25519 and no gpgme
-            image: debian:buster-slim
+          - name: Debian Stable with curl, sign-ed25519 and no gpgme
+            image: debian:stable-slim
             pre-checkout-setup: |
               apt-get update
               apt-get install -y git
@@ -59,8 +64,8 @@ jobs:
 
           # A 32 bit build to act as a proxy for frequently deployed 32
           # bit armv7
-          - name: Debian Buster (10) 32 bit
-            image: i386/debian:buster-slim
+          - name: Debian Stable 32 bit
+            image: i386/debian:stable-slim
             # This is pretty nasty. The checkout action uses an x86_64
             # node binary in the container, so we need to provide an
             # x86_64 ld.so and libstdc++.
@@ -69,14 +74,26 @@ jobs:
               apt-get update
               apt-get install -y git libc6:amd64 libstdc++6:amd64
 
-          - name: Ubuntu Focal (20.04)
-            image: ubuntu:focal
+          - name: Debian Testing
+            image: debian:testing-slim
             pre-checkout-setup: |
               apt-get update
               apt-get install -y git
 
-          - name: Ubuntu Groovy (20.10)
-            image: ubuntu:groovy
+          # Ubuntu builds. Unfortunately, when the latest release is
+          # also the latest LTS, latest and rolling are the same. Other
+          # options would be to test the previous LTS by name or to test
+          # the devel tag, which is the unreleased version.
+          #
+          # https://hub.docker.com/_/ubuntu
+          - name: Ubuntu Latest LTS
+            image: ubuntu:latest
+            pre-checkout-setup: |
+              apt-get update
+              apt-get install -y git
+
+          - name: Ubuntu Latest Release
+            image: ubuntu:rolling
             pre-checkout-setup: |
               apt-get update
               apt-get install -y git