(maint) Drop debian-10 from testing matrix

Debian 10 was eol last year. Packages have now been dropped from the
main deb.debian.org mirrors and are now available only from
archive.debian.org, so the images no longer work correctly.

Author: jpartlow

.github/workflows/pr_testing.yaml

@@ -56,7 +56,6 @@ jobs:
           - almalinux:9
           - rockylinux:8
           - rockylinux:9
-          - debian:10
           - debian:11
           - debian:12
           - fedora:42

.github/workflows/pr_testing_install_build_artifact.yaml

@@ -41,7 +41,6 @@ jobs:
           - almalinux:9
           - rockylinux:8
           - rockylinux:9
-          - debian:10
           - debian:11
           - debian:12
           - fedora:42

.github/workflows/pr_testing_with_nested_vms.yml

@@ -16,7 +16,6 @@ jobs:
         os:
           - [almalinux, '8']
           - [almalinux, '9']
-          - [debian, '10']
           - [debian, '11']
           - [debian, '12']
           - [debian, '13', 'amd64', 'daily-latest']