[Bridge] [Swiftmailer] adds deprecation notice on MessageDataCollector class.
minor #13056 [2.7] [Bridge] [Swiftmailer] adds deprecation notice on MessageDataCollector class. (hhamon)

This PR was merged into the 2.7 branch.

Discussion
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[2.7] [Bridge] [Swiftmailer] adds deprecation notice on MessageDataCollector class.

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

Commits
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c003bb1 [Bridge] [Swiftmailer] adds deprecation notice on MessageDataCollector class.