minor #38460 Remove "version" from composer.json files, use "branch-version" instead (nicolas-grekas)

This PR was merged into the 3.4 branch.

Discussion
----------

Remove "version" from composer.json files, use "branch-version" instead

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Waiting for confirmation from @Seldaek or @naderman

Commits
-------

f9ed6940fd Remove "version" from composer.json files, use "branch-version" instead

Author: nicolas-grekas

composer.json

@@ -58,5 +58,7 @@
         ]
     },
     "minimum-stability": "dev",
-    "version": "3.4.x-dev"
+    "extra": {
+        "branch-version": "3.4-dev"
+    }
 }