[backport to release/3.9.x] chore(cd): remove EOL distros

[Water-Melon]

KAG-8998

#14873
(cherry picked from commit d4c6d9a8bb479b2dae59f781c068c3ccd9db6180)

Summary

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/developer.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]

[Copilot]

Pull request overview

Backports the CI/CD matrix update to stop building and releasing packages for EOL distributions on the release/3.9.x branch, reducing maintenance and pipeline workload.

Changes:

• Removed Ubuntu 20.04 from build-packages and release-packages matrices.
• Removed RHEL 8 (Rocky 8 / el8) from build-packages and release-packages matrices.

Comments suppressed due to low confidence (2)

.github/matrix-full.yml:13

• With ubuntu-20.04 removed from this matrix, the corresponding manifest expectation suite ("ubuntu-20.04-amd64") in scripts/explain_manifest/config.py becomes unused along with its fixture. Consider removing those entries (or documenting why they remain) to avoid stale/unused manifest config drifting over time.

# check-manifest-suite: the check manifest suite as defined in scripts/explain_manifest/config.py

# Ubuntu
- label: ubuntu-22.04
  image: ubuntu:22.04
  package: deb
  check-manifest-suite: ubuntu-22.04-amd64

.github/matrix-full.yml:44

• Now that the rhel-8 build/release targets are removed from this matrix, scripts/explain_manifest/config.py still defines an "el8-amd64" suite/fixture that appears unused. Consider cleaning it up (or adding a note) to prevent dead manifest suites from accumulating.

# RHEL
- label: rhel-9
  image: rockylinux:9
  package: rpm
  package-type: el9
  check-manifest-suite: el9-amd64

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