minor #18815 Update blackfire free plan explanation (alamirault)

This PR was merged into the 5.4 branch.

Discussion
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Update blackfire free plan explanation

Blackfire removed free plan but our doc is not up to date (reported in symfony slack)

https://www.blackfire.io/pricing/#free-trial

Commits
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3d4fc32 Update blackfire free plan explanation

Author: javiereguiluz

performance.rst

@@ -224,7 +224,7 @@ Profiling with Blackfire
 
 `Blackfire`_ is the best tool to profile and optimize performance of Symfony
 applications during development, test and production. It's a commercial service,
-but provides free features that you can use to find bottlenecks in your projects.
+but provides a `Blackfire demonstration`_.
 
 Profiling with Symfony Stopwatch
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -353,5 +353,6 @@ Learn more
 .. _`cachetool`: https://github.com/gordalina/cachetool
 .. _`open_basedir`: https://www.php.net/manual/ini.core.php#ini.open-basedir
 .. _`Blackfire`: https://blackfire.io/docs/introduction?utm_source=symfony&utm_medium=symfonycom_docs&utm_campaign=performance
+.. _`Blackfire demonstration`: https://demo.blackfire.io?utm_source=symfony&utm_medium=symfonycom_docs&utm_campaign=performance
 .. _`Stopwatch component`: https://symfony.com/components/Stopwatch
 .. _`real-world stopwatch`: https://en.wikipedia.org/wiki/Stopwatch