minor #14221 Fix symfony/form dependend components (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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Fix symfony/form dependend components

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Because both the FrameworkBundle and the Doctrine bridge use the new choice list feature of 2.7, they are incompatible with Form below 2.7

Commits
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278f5c9 Fix symfony/form dependend components

src/Symfony/Component/Form/composer.json

@@ -31,6 +31,10 @@
         "symfony/security-csrf": "~2.4|~3.0.0",
         "symfony/translation": "~2.0,>=2.0.5|~3.0.0"
     },
+    "conflict": {
+        "symfony/doctrine-bridge": "<2.7",
+        "symfony/framework-bundle": "<2.7"
+    },
     "suggest": {
         "symfony/validator": "For form validation.",
         "symfony/security-csrf": "For protecting forms against CSRF attacks.",