docs: bump release stamps to 2.8.0

[orieg]

Summary

Close the version stamps the 2.8.0 release (#226) left behind. Docs only.

File	Change
SECURITY.md	supported-versions table: 2.8.x ✅ / < 2.8 ❌ (was 2.7.x / < 2.7)
docs/index.html	hero badge → Release v2.8.0 Live (was v2.7.1)

Deliberately left as historical references:

• docs/CI_CD_GUIDE.md's v2.7.1 rate-limit / downloads.php.net incident note.
• docs/index.html's "What's New in v2.7.0" spotlight and the TTL-caching code tabs — they describe the STRING_TO_ENTRY feature introduced in 2.7.0.

Not touched: BENCHMARK.md's footer Describes line stays at its #217-reconciled value (2.5.2). Bumping it to 2.8.0 would re-assert a measurement that does not exist — the figures are measured on 2.4.2 and re-verified through 2.5.2. Easy to add if you want the release-number convention instead.

[github-actions]

discipline: passed

Base main (origin/main) (merge base f0e4deaf33) · 27 gate(s) passed · 2164 item(s) examined · 0 warning(s) · 0 override(s)

_{discipline explain <gate> prints what a gate checks and how a finding is lifted. This comment is updated on every run; the check's status is the verdict.}