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>

rpms/vminsert/metadata.yaml

@@ -9,11 +9,6 @@ homepage: https://github.com/VictoriaMetrics/VictoriaMetrics
 license: Apache-2.0 license
 dist_tag: true
 builds:
-- repository:
-  - almalinux/el8
-  image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
-  release: 1
-  version: 1.142.0
 - repository:
   - almalinux/el9
   image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/rpmbuilder:0.1.0-alma9