minor #15520 [FrameworkBundle] Deprecate services in Doctrine cache (javiereguiluz)

This PR was merged into the 5.3 branch.

Discussion
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[FrameworkBundle] Deprecate services in Doctrine cache

This fixes the first part of #15505.

Commits
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675b2ac [FrameworkBundle] Deprecate services in Doctrine cache

Author: OskarStark

reference/configuration/framework.rst

@@ -2801,6 +2801,10 @@ none
 a service id
     A service id referencing a `Doctrine Cache`_ implementation
 
+.. deprecated:: 5.3
+
+    Using a service ID as the value of ``cache`` is deprecated since Symfony 5.3.
+
 file_cache_dir
 ..............