fix: resolve GTNH server packs from versions.json

[WowSuchRicky]

Summary

Fix GTNH server-pack discovery against the current downloads.gtnewhorizons.com/versions.json metadata endpoint.

Changes

• Normalize stable, beta/RC, and exact-version selections to one JSON shape.
• Include RC builds in GTNH_PACK_VERSION=latest-dev.
• Select Java 8 versus Java 17+ server URLs from release metadata.
• Enforce maxJavaVersion and reject missing/non-HTTPS server URLs.
• Add deterministic fixture-based regression tests for wrapped and direct metadata forms.
• Document the selection and Java compatibility behavior.

Reproduction

The container's GTNH startup path must resolve server files from versions.json. The implementation must not produce the previous Server files not found failure for latest-dev or exact versions.

Validation

• bash -n scripts/start-deployGTNH tests/gtnh-versions-json/test.sh
• shellcheck -x scripts/start-deployGTNH
• shellcheck tests/gtnh-versions-json/test.sh
• tests/gtnh-versions-json/test.sh
• git diff --check

The repository-wide Docker test harness was not runnable locally because Docker is unavailable; please use CI for the full integration gate.

[WowSuchRicky]

Apologies this should have still been a draft. Will reopen at another time or resubmit.