Remove ubuntu20 runner

[int128]

This removes the outdated Ubuntu 20.04 runner.

[github-actions]

💡 If you need a new version, create a new release after merge.

[Copilot]

Pull request overview

Removes the Ubuntu 20.04 runner variant and simplifies the CI/release workflows to build/test/release a single runner image.

Changes:

• Removed the Ubuntu 20.04 Dockerfile and related “ubuntu20” build flavor.
• Simplified reusable workflows by collapsing runner-image-uri-latest/runner-image-uri-ubuntu20 into a single runner-image-uri input.
• Updated E2E workflow to create and validate only one runner scale set (plus a dummy one for job pickup).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Marks Ubuntu 20.04 runner support as deprecated/removed in the feature list.
Dockerfile.ubuntu20	Deletes the Ubuntu 20.04 runner image definition.
.github/workflows/reusable--release.yaml	Removes dual-image version parity check; uses a single runner image input.
.github/workflows/reusable--e2e-test.yaml	Removes ubuntu20 runner provisioning/testing; switches to a single runner image input.
.github/workflows/reusable--e2e-test-jobs.yaml	Removes ubuntu20-specific conditional skips for Ruby steps.
.github/workflows/build.yaml	Removes the ubuntu20 build job and rewires e2e-test/release to depend on the single build job.

Comments suppressed due to low confidence (1)

Dockerfile.ubuntu20:1

• The RUN step that downloads docker-${DOCKER_VERSION}.tgz via curl from https://download.docker.com/... and immediately extracts/installs it introduces a supply‑chain risk, because the binary is trusted and executed without any checksum or signature verification.
  If an attacker were ever able to compromise download.docker.com or the TLS channel, they could deliver a malicious Docker binary that would be installed and later run with high privileges in your runner environment.
  To harden this, fetch and verify the official checksum or signature for the Docker archive (or use a base image that already contains Docker from a trusted registry) before installation so the build fails on tampering.

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