The `update-release-calendar.sh` script did not account for Extended
Support Release (ESR) versions. Running it would drop ESR entries (e.g.
2.24) from the calendar entirely or mark them as "Not Supported" instead
of "Extended Support Release".
## Changes
- Add `ESR_VERSIONS` array for tracking active ESR minor versions
- Add `is_esr_version()` helper to check ESR membership
- Extract `generate_release_row()` to reduce duplication
- Prepend ESR versions older than the standard window
- Override "Not Supported" status for ESR versions within the window
> [!NOTE]
> When new ESR versions are designated or old ones reach end of life,
update the `ESR_VERSIONS` array at the top of the script.
<!-- This PR was authored by Coder Agents -->
Garrett Delfosse