docs: update testnet version history for v1.8.0-rc1 upgrade

[eungu0920]

Summary

Docs i18n checklist

• Edited docs/pages/en/ only (did not hand-edit cn/ko content)
• Updated docs/sidebar.json (/en/) if pages were added, moved, or renamed
• Reviewed the auto-generated cn/ko translations committed to this PR
• If a page was deleted/renamed: applied the same change to cn and ko
• npm run i18n:check reports 0 missing (stale warnings OK)

[github-actions]

Styleguide check

✓ 132 en pages — 0 blocking, 11 warnings

docs/pages/en/explanation/autobahn.mdx

• ⚠ warning: marketing word "robust" (L20)
• ⚠ warning: marketing word "seamless" (L34)

docs/pages/en/explanation/consensus.mdx

• ⚠ warning: marketing word "robust" (L32)

docs/pages/en/explanation/ethereum-compatibility.mdx

• ⚠ warning: marketing word "seamless" (L10)

docs/pages/en/how-to/monitor-node.mdx

• ⚠ warning: marketing word "comprehensive" (L7)

docs/pages/en/how-to/troubleshoot-node.mdx

• ⚠ warning: marketing word "comprehensive" (L7)

docs/pages/en/reference/indexers.mdx

• ⚠ warning: marketing word "seamlessly" (L9)

docs/pages/en/reference/ramps.mdx

• ⚠ warning: marketing word "seamless" (L28)

docs/pages/en/reference/system-transactions-api.mdx

• ⚠ warning: marketing word "comprehensive" (L358)

docs/pages/en/reference/wallets.mdx

• ⚠ warning: marketing word "powerful" (L83)
• ⚠ warning: marketing word "seamless" (L30)

_{Enforced by npm run style:check — see STYLEGUIDE.md.}