bug #12274 Fix no identifier defined error in ApiPlatform 2.6 (kayue)

This PR was merged into the 1.8 branch.

Discussion
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Caused by this change api-platform/core#3871

| Q               | A
| --------------- | -----
| Branch?         | 1.8
| Bug fix?        | yes
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | api-platform/core#3871
| License         | MIT

Commits
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5a7bc59 Fix no identifier defined error in ApiPlatform 2.6

src/Sylius/Bundle/ApiBundle/Resources/config/api_resources/Commands/RegisterShopUser.xml

@@ -40,7 +40,7 @@
 
         <property name="firstName" required="true" />
         <property name="lastName" required="true" />
-        <property name="email" required="true" />
+        <property name="email" required="true" identifier="true" />
         <property name="password" required="true" />
     </resource>
 </resources>