[2.3] Updated the swiftmailer version constraint

| Q             | A
| ------------- | ---
| Bug fix?      | no
| 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 `swiftmailer/swiftmailer` version constraint in the 2.3 branch.

`"~4.2|~5.0"` is far cleaner than `">=4.2.0,<6.0-dev"`, and does the same thing.

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This accompanies #12832 on the 2.5 branch.