Template current-release versions in download pages

[renatsaf]

Summary

Phase 1 of #25503. Replaces the hardcoded current-release version literals in the download pages with the existing ${glassfish.version.*} properties from docs/pom.xml, which are already resolved by the website module's resource filtering. This removes the per-release copy&paste for the latest 7.x, 7.1.x and 8.x entries.

• download.md: 8.0.2 → ${glassfish.version.8x}, 7.1.0 → ${glassfish.version.71x}
• download_gf7.md: 7.1.0 → ${glassfish.version.71x}, 7.0.25 → ${glassfish.version.7x}
• download_gf8.md: 8.0.2 → ${glassfish.version.8x}

Rendered output is unchanged: the properties already resolve to these exact values in the build (the existing target/classes/docs/README.md confirms ${glassfish.version.71x} → 7.1.0).

Scope / why this is only Phase 1

Older release blocks (8.0.1, 8.0.0, 7.0.24 and below) are left as literals on purpose. Maven property filtering is global/static, so a property can only carry the current value of each series — only the latest entry per series maps to a property. The repeated historical blocks need a real templating engine with per-block variable reassignment (Velocity/FreeMarker, as discussed in the issue), which is left for a follow-up. Hence Refs rather than Fixes.

Notes for reviewers

• Opened as draft to coordinate with the mentored work already in progress on Support templating engine in download pages #25503 (cc @OndroMih / @Ariho-Seth) — happy to defer, hand off, or fold this in as a starting point.
• No copyright headers were added to these .md files: they currently carry none and render directly into the published website HTML, so a header would leak into the page. Please confirm website content .md is treated like .adoc (exempt).

Refs #25503

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