chore: update GreptimeDB documentation version to v1.2.0

[discord9]

What changed

Update only variables/variables-1.2.ts, setting greptimedbVersion from v1.2.0-beta.2 to v1.2.0 with the existing .github/scripts/release-patch.sh 1.2.0 script.

Merge effect: this promotes documentation version 1.2 to the default site root for both languages, removes its pre-release banner, and changes canonical/sitemap routing. This is a proposed change only; merge timing must be coordinated with the release owner. GitHub v1.2.0 still remains Pre-release, and this PR does not change GitHub Latest.

The 1.2 versioned documentation already exists: no new-minor generator was run and no version snapshots, sidebars, or navigation files were changed.

Release notes are separate: #2839
Release: https://github.com/GreptimeTeam/greptimedb/releases/tag/v1.2.0
Tracking: GreptimeTeam/greptimedb#9015

Scope

• Documentation versions: shared 1.2 release variable and resulting default-version routing.
• Languages: English and Chinese.

Verification

• git diff --check: passed.
• pnpm test: passed (39 tests in 6 files, including site-version routing tests).
• DOC_LANG=en pnpm check:links: passed.
• DOC_LANG=zh pnpm check:links: passed.
• pnpm typecheck: fails with missing @docusaurus/theme-classic and node type definitions in this environment. The same two errors were reproduced on the unchanged-dependency notes branch; no dependency or tsconfig changes were added to this version-only PR.
• Reviewed resolveLastVersion and confirmed that this variable makes 1.2 the root version. No website deployment or merge was performed locally.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed. (Not applicable: no structural change; version routing derives from the shared variable.)

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