minor #17798 update CI intro (alexislefebvre)

This PR was merged into the 5.4 branch.

Discussion
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update CI intro

> The Symfony project uses a third-party service

It was true when tests where ran by Travis CI.

Now tests are run by GitHub Actions, which is not a third-party from GitHub's viewpoint.

I suggest a more generic approach: there is a CI service, no matter if it's internal or third-party.

Commits
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03995aa update CI intro

Author: javiereguiluz

contributing/code/tests.rst

@@ -3,7 +3,7 @@
 Running Symfony Tests
 =====================
 
-The Symfony project uses a third-party service which automatically runs tests
+The Symfony project uses a CI (Continuous Integration) service which automatically runs tests
 for any submitted :doc:`patch <pull_requests>`. If the new code breaks any test,
 the pull request will show an error message with a link to the full error details.