fix(create-release): replace mindsers/changelog-reader-action with inline awk

[weatherhog]

Summary

Replaces mindsers/changelog-reader-action@v2.2.3 in the create-release reusable workflow with an inline awk extractor.

The action is unmaintained (no release since March 2024) and still runs on Node 20, which GitHub Actions is deprecating on 2026-06-02. Inline shell has no Node runtime, so the workflow stops being exposed to future Node-deprecation cycles.

Refs: giantswarm/giantswarm#36091

Why inline awk instead of forking the action

• The action is used in exactly one place (this reusable workflow); every consumer benefits transitively.
• The job is a trivial text extraction on Keep-a-Changelog files; the upstream TypeScript adds no algorithmic value for how it's used here.
• Forking solves today's Node 20→24 problem but recreates it in ~2 years on the next Node deprecation, plus carries org-fork governance overhead.
• The other Node-20 action flagged in the issue, ncipollo/release-action, was already bumped to v1.21.0 (Node 24) by Renovate in Update ncipollo/release-action action to v1.21.0 #138 — so this PR closes the remaining work for #36091.

Validation

Tested the new awk extractor against the actual GitHub Release bodies that the existing workflow produced for every release published after 2026-02-06 (the date this reusable workflow was introduced) across 100 repos that consume it — 178 releases in total.

After filtering out 35 releases from 4 repos (kserve, vllm, mcp-debug, mcp-oauth) that use a different release flow with GitHub-auto-generated release notes (i.e. weren't produced by mindsers/changelog-reader-action at all), the result is:

128 / 128 release bodies match byte-for-byte.

(One visible per-line trailing-space difference in flux-app v1.9.0 is the result of GitHub's API normalizing trailing whitespace server-side when storing release bodies — the stored body is identical either way; the upstream action wouldn't strip it either.)

The awk handles three Keep-a-Changelog edge cases that the upstream action also handled:

• Stops at the next ## heading (section boundary).
• Skips Markdown reference-link definitions ([X]: https://...) — matters only for first releases of a repo, where the link block sits glued to the section.
• Trims leading and trailing blank lines (the action's outer .trim() does the same).

If the requested version isn't found in the file, the step fails fast with ::error::No changelog entry found for version ..., preserving the action's "fail the build on missing entry" behavior.

Test plan

• YAML parses (python3 -c 'import yaml; yaml.safe_load(open(...))').
• Byte-for-byte comparison against 128 historical releases (above).
• Smoke test by triggering a real patch release in a low-stakes consumer repo (e.g. giantswarm/debug or giantswarm/to) after merge, and visually comparing the resulting Release body to the prior release.