chore: bump json to 2.21.1 to fix low-severity CVE-2026-54696

- json 2.19.3 -> 2.21.1 (low, CVE-2026-54696: heap buffer overflow when
  streaming to an IO); surfaced by bundle-audit's Security Audit CI job,
  not yet a GitHub Dependabot alert on this repo

Signed-off-by: Jonathan Norris <jonathan.norris@dynatrace.com>

Author: jonathannorris

Gemfile.lock

@@ -52,7 +52,7 @@ GEM
       prism (>= 1.3.0)
       rdoc (>= 4.0.0)
       reline (>= 0.4.2)
-    json (2.19.3)
+    json (2.21.1)
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
     listen (3.10.0)