Simplify GitHub Actions workflows

[0x5143]

Motivation

• Reduce maintenance burden by consolidating CI and docs verification into the main Build workflow and keep only the essential Release workflow.
• Remove short-lived or ancillary workflows and a small custom composite action that added complexity but little long-term value.
• Ensure triggers are intuitive by letting the central Build workflow cover changes to documentation and .github instead of scattering checks across several small workflows.

Description

• Deleted the following workflows and custom action: .github/workflows/build-docs.yml, .github/workflows/publish-docs.yml, .github/workflows/prevent-github-change.yml, .github/workflows/pr-merged.yaml, .github/workflows/publish-backplane-redis.yml, .github/workflows/stale.yml, and .github/actions/check-metas/action.yaml.
• Updated .github/workflows/build.yml to remove paths-ignore (so docs and .github changes are covered) and to use the local composite action ./.github/actions/setup-dotnet in place of the external reusable action.
• Updated .github/workflows/release.yml to use the local ./.github/actions/setup-dotnet where the job performs a checkout, and removed the now-deleted check-metas step and actions-timeline usage.
• Committed changes with message Simplify GitHub Actions workflows and prepared the PR metadata (make_pr) summarizing the simplification.

Testing

• Ran a Ruby YAML loader over remaining .github YAML files via ruby -e 'require "yaml"; ...' which succeeded and validated YAML syntax.
• Performed repository searches with rg to assert removed workflow names and internal action references are no longer present, which succeeded.
• Verified repository state with git status / git diff --stat and created a commit via git commit -m "Simplify GitHub Actions workflows", which succeeded.
• Generated PR metadata with make_pr to capture the change summary and testing notes, which succeeded.

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Codex Task