…ailable (Raphaël Droz)

This PR was squashed before being merged into the 6.2 branch.

Discussion
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SMTP Transport to provide the (final) Message-ID if available

bug #46323 [Mailer] SMTP transport does not provide the (final) Message-ID

| Q             | A
| ------------- | ---
| Branch?       | 6.1
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #46323
| License       | MIT
| Doc PR        | symfony/symfony-docs#

Questions / Answers:
- **Q**: Why storing 250 inside **`$mtaResult`** instead of using the already available `$message->getDebug()`?
- A: Because the later may be huge (especially in case of attachment) and running a regexp over it would be less than safe. Instead `$mtaResult` only contains one line
- **Q**: Why a new **`public`** method?
- A: Because child SMTP Transport may make other assumptions about the return `250 Ok` value and provide a custom logic / regexp.
- **Q**: Why **regexp**?
- A: Because there is no official standard for this line (other than the `250 Ok` prefix)

Additionally (see https://symfony.com/releases):
- [ ] Always add tests and ensure they pass. _I can't. Don't have the time. Left to maintainers / other contributors_
- [x] Bug fixes must be submitted against the lowest maintained branch where they apply (lowest branches are regularly merged to upper ones so they get the fixes too.)
- [x] Features and deprecations must be submitted against the latest branch.
- [x] Changelog entry should follow https://symfony.com/doc/current/contributing/code/conventions.html#writing-a-changelog-entry
- [x] Never break backward compatibility (see https://symfony.com/bc).

Commits
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441c9b0 SMTP Transport to provide the (final) Message-ID if available