This PR was merged into the 4.4 branch.

Discussion
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[Mailer] Simplify the way TLS/SSL/STARTTLS work

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | -

The way TLS/SSL/STARTTLS is handled is not easy to understand. It's inherited from Swiftmailer and today, I've spent some time to see if we could improve it.

First, the current way: `setEncryption()` takes a string, either `ssl` or `tls`:

 * `ssl`: to enable TLS support on the connection
 * `tls`: to enable `STARTTLS` (upgrade the connection)

There is also a `isTLS()` method which is really confusing due to the fact that both configuration are about TLS anyway.

So, this PR changes things radically:

 * The `setEncryption` method and the `encryption` option on the DSN are gone.

 * TLS is used by default and you can disable it via `disableTls()`. Being secure by default is probably a good idea anyway (like using HTTPS by default instead of HTTP).

 * A new "protocol" SMTPS is supported now and is a way to say that you want TLS; so use `smtps://localhost` to set TLS instead of `smtp://localhost?encryption=ssl`. Note that using `smtp://localhost:465` does the same. All third-party providers now supports both `smtp` and `smtps` protocol even if that does the exact same thing for them (TLS is always enabled).

 * The port is automatically determined based on the TLS setting (if not set explicitly). So 465 for TLS and falls back to 25.

 * There is no more way to enable `STARTTLS`. If you don't configure TLS on the connection and if the server supports `STARTTLS`, then we will enable it automatically.

Great document about all of this: https://www.fastmail.com/help/technical/ssltlsstarttls.html

Commits
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5b8c467 [Mailer] simplified the way TLS/SSL/StartTls work