Drop el8 builds (#188)

el8 has no builder image any more (the old almalinux8-rpmbuilder image can't be pulled) and el8 packages are no longer wanted.

- remove build/deploy almalinux8 pipelines
- remove el8 build entries from all package metadata
- drop almalinux/el8 from the metadata schema and tools/build

Requires unkin/terraform-git#110 applied (removes the build-almalinux8 required check).

Reviewed-on: #188
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

schema/metadata.json

@@ -34,7 +34,6 @@
           "repository": {
             "items": {
               "enum": [
-                "almalinux/el8",
                 "almalinux/el9",
                 "fedora/42",
                 "fedora/43",