minor #10082 Doctrine server_version should always be a string (alexander-schranz)

This PR was merged into the 2.8 branch.

Discussion
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Doctrine server_version should always be a string

As the comment above say the server_version should always be a string and wrapped by quotes.

Commits
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1bb4d26 server_version should always be a string

Author: javiereguiluz

reference/configuration/doctrine.rst

@@ -136,13 +136,13 @@ If you want to configure multiple connections in YAML, put them under the
                     user:             root
                     password:         null
                     host:             localhost
-                    server_version:   5.6
+                    server_version:   '5.6'
                 customer:
                     dbname:           customer
                     user:             root
                     password:         null
                     host:             localhost
-                    server_version:   5.7
+                    server_version:   '5.7'
 
 The ``database_connection`` service always refers to the *default* connection,
 which is the first one defined or the one configured via the