Add additional CI jobs to avoid accidental breakage

In general, we'd like to make sure Git works on the LTS versions of
major Linux distributions.  To do that, let's add CI jobs for the oldest
regular (non-extended) LTS versions of the major distributions: Ubuntu
20.04, Debian 11, and RHEL 8.  Because RHEL isn't available to the
public at no charge, use AlmaLinux, which is binary compatible with it.

Note that Debian does not offer the language-pack packages, but suitable
locale support can be installed with the locales-all package.
Otherwise, use the set of installation instructions which exist and are
most similar to the existing supported distros.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

.github/workflows/main.yml

@@ -342,12 +342,21 @@ jobs:
         - jobname: linux-musl
           image: alpine
           distro: alpine-latest
+        # Supported until 2025-04-02.
         - jobname: linux32
           image: i386/ubuntu:focal
           distro: ubuntu32-20.04
         - jobname: pedantic
           image: fedora
           distro: fedora-latest
+        # A RHEL 8 compatible distro.  Supported until 2029-05-31.
+        - jobname: almalinux-8
+          image: almalinux:8
+          distro: almalinux-8
+        # Supported until 2026-08-31.
+        - jobname: debian-11
+          image: debian:11
+          distro: debian-11
     env:
       jobname: ${{matrix.vector.jobname}}
       distro: ${{matrix.vector.distro}}