[SPARK-58371][DOC] Update json gem version to 2.21.1

[LuciferYang]

What changes were proposed in this pull request?

This PR upgrades the json gem from 2.12.2 to 2.21.1 in docs/Gemfile.lock. It is a transitive dependency (pulled in by jekyll, which requires json (~> 2.6)), and 2.21.1 satisfies that constraint, so only the locked spec version changes.

json has no runtime dependencies, so no other lock entries change and docs/Gemfile does not need to be touched.

Why are the changes needed?

2.21.1 includes the fix for a security advisory that affects all versions in >= 2.9.0, < 2.19.9:

• GHSA-x2f5-4prf-w687 / CVE-2026-54696 (low): heap out-of-bounds write in the JSON generator when streaming to an IO. On the IO path, fbuffer_do_inc_capa() compared the requested size against the buffer's total capacity instead of its remaining capacity, so JSON.dump(obj, io) or JSON::State#generate(obj, io) could write past the buffer when serializing a string near 16 KB. Fixed in 2.19.9.

Following the same pattern as SPARK-57633 (concurrent-ruby 1.3.7), this picks up the latest release rather than the minimum patched version.

Does this PR introduce any user-facing change?

No. This only affects the documentation build toolchain.

How was this patch tested?

Manually verified in a scratch copy of docs/Gemfile and docs/Gemfile.lock:

1. Lock resolution matches. Running bundle lock --update=json produces a lock file byte-identical to the hand-edited one, confirming 2.21.1 satisfies jekyll's json (~> 2.6) and that no other locked spec is affected.

2. Frozen install succeeds. BUNDLE_FROZEN=true bundle install installs all 36 gems without modifying the lock file, and bundle list reports json (2.21.1).

3. The advisory's PoC no longer crashes:

require "json"
require "stringio"

io = StringIO.new
big = "a" * 16385
big[16382] = '"'          # escapable byte near the buffer boundary

JSON.dump([big], io)

json version: 2.21.1
PoC (JSON.dump with IO): no crash, output bytes = 16390
roundtrip ok: true

4. The docs site builds:

$ cd docs && SKIP_API=1 bundle exec jekyll build
Configuration file: .../docs/_config.yml

************************
* Building error docs. *
************************
Generated: docs/_generated/error-conditions.html
            Source: .../docs
       Destination: .../docs/_site
 Incremental build: disabled. Enable with --incremental
      Generating...
Warning: Tolerating missing API files because the following skip flags are set: SKIP_API
                    done in 3.125 seconds.
 Auto-regeneration: disabled. Use --watch to enable.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 5)

[dongjoon-hyun]

+1, LGTM.

[LuciferYang]

Merge Summary:

• merged into master 014c53b

Posted by merge_spark_pr.py

[LuciferYang]

Thank you @dongjoon-hyun