centralize version list and pin external dependencies

[schwzr]

Consolidates all version lists into releases.ts and pins external dependencies.

Centralized version list

• releases.ts is the single source of truth for all 19 versions, sidebar
  mapping, latest version, and image digests
• Version dropdown, sidebar routing, navbar label, and old-versions page all
  derive from it — no duplicate lists
• fetch-versioned-release-notes.sh replaced with TypeScript version that
  imports allVersions from releases.ts
• OldVersions.vue component replaces hand-maintained Markdown link list
• create-version.ts script automates new version creation (folder copy,
  releases.ts update, symlink, release notes fetch)

Dependency pinning

• GitHub Actions pinned to full SHA commits with version comments
• npm dependencies pinned to exact versions (removed ^ ranges)
• Dependabot config for weekly update PRs (actions + npm)

[schwzr]

• Check for action version updates
• Minimize action permissions