Fixed the symfony/config version constraint

Fixes the symfony/config version constraint in the proxy manager bridge
minor #12781 Fixed the symfony/config version constraint (GrahamCampbell)

This PR was merged into the 2.3 branch.

Discussion
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Fixed the symfony/config version constraint

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

##### This pull request fixes the `symfony/config` version constraint in the proxy manager bridge.

Before this pull, composer was only allowing `v2.3.0` to be downloaded. This surely was not the intention. I've updated the version constraint from `"2.3"` to `"~2.3"` thus allowing all 2.x versions that are equal to or greater than `v2.3.0`.

Commits
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ec14f0f Fixed the symfony/config version constraint