build(deps): bump mini_racer from 0.20.0 to 0.21.0 in /docs/v3 (#5073)

Bumps [mini_racer](https://github.com/discourse/mini_racer) from 0.20.0 to 0.21.0.
- [Changelog](https://github.com/rubyjs/mini_racer/blob/main/CHANGELOG)
- [Commits](rubyjs/mini_racer@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: mini_racer
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/v3/Gemfile.lock

@@ -108,7 +108,7 @@ GEM
     middleman-syntax (3.6.1)
       middleman-core (>= 3.2)
       rouge (~> 3.2)
-    mini_racer (0.20.0)
+    mini_racer (0.21.0)
       libv8-node (~> 24.12.0.1)
     minitest (6.0.2)
       drb (~> 2.0)