Add RELEASE_VERSION env to the images for 8.x series

[Peter-Sh]

Related to #518 (but for newer versions)

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Note

Low Risk
Low risk: build metadata/templating changes that only add a new env var and JSON field, with minimal impact on runtime behavior.

Overview
Adds release_tag to .redis.version.json and updates the version-apply GitHub Action script to write it when bumping Redis.

Updates the Alpine and Debian Dockerfile Jinja templates (and regenerated Dockerfiles) to set ENV REDIS_VERSION={{ release_tag }} when provided, and extends the stackbrew Dockerfile rendering context to accept release_tag.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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