minor #18980 Update the docs-builder to 0.21 (SzymonKaminski)

This PR was merged into the 5.4 branch.

Discussion
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Update the docs-builder to 0.21

The website seems to be running docs-builder v0.21 or 0.20.6 (autoscrolling of `:method:` links works properly in Chromium-based browsers/Safari), but the repository still has v0.20.5.

Diff: symfony-tools/docs-builder@v0.20.5...v0.21.0

The `tabs` directive is documented at https://symfony.com/doc/current/contributing/documentation/format.html#displaying-tabs, but is not yet used anywhere in the actual docs, so the build is not failing with current version of docs-builder, nonetheless the `:method:` links generated locally are incorrect.

Commits
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f436f08 Update the docs-builder to 0.21

Author: javiereguiluz

_build/composer.json

@@ -17,6 +17,6 @@
         "php": ">=8.1",
         "symfony/console": "^6.2",
         "symfony/process": "^6.2",
-        "symfony-tools/docs-builder": "^0.20"
+        "symfony-tools/docs-builder": "^0.21"
     }
 }