Update locked json gem to 2.21.2 to unblock Dependabot security updates

[Copilot]

Dependabot was failing on the Bundler update job because the repository lockfile resolved json to 2.20.0, while the first non-vulnerable version is 2.21.2. This change updates the locked transitive dependency so Dependabot can produce a valid security update.

• Root cause

  • Gemfile.lock pinned json at 2.20.0
  • Dependabot reported security_update_not_possible because 2.20.0 was the latest resolvable version in the existing lock state, but not a patched one

• Change

  • Bumped the locked json gem from 2.20.0 to 2.21.2
  • Kept the Gemfile unchanged since dependency management already flows through github-pages; only the resolved lock state needed correction

• Scope

  • No site content, Jekyll config, or workflow logic changes
  • Single-file update in Gemfile.lock

-    json (2.20.0)
+    json (2.21.2)