[SwiftmailerBridge] Bump allowed versions of swiftmailer

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes (but SwiftmailerBridge itself does not contain any tests)
| Fixed tickets | -
| License       | MIT
| Doc PR        | -


Current version of Swiftmailer is 5.2.1, while (previously to this commit)
the version installed by composer was 5.0.3.

This is rather important, since 5.2.1 closes a security issue that 5.0.3 is
vulnarable to (https://github.com/swiftmailer/swiftmailer/issues/494).