This PR was merged into the 4.4 branch.

Discussion
----------

[Mailer] Change the DSN semantics

| 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    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#12258

I'm not very satisfied with the current DSNs for the Mailer. First, the scheme/protocol should use the provider name, then, there is no way to change the host, which would be nice to debug more easily (using a requestb.in service for instance).

Before:

```
smtp://USERNAME:PASSWORD@mailgun
http://KEY:DOMAIN@mailgun
```

After:

```
mailgun+smtp://USERNAME:PASSWORD@default
mailgun+http://KEY:DOMAIN@default

# New
mailgun+http://KEY:DOMAIN@somewhere.com:99
```

SMTP DSNs did not change, but the special sendmail one did:

Before:

```
smtp://sendmail
```

After:

```
sendmail+smtp://default
```

And for the `null` transport:

Before:

```
smtp://null
```

After:

```
null://null
```

Commits
-------

469c989 [Mailer] Change the DSN semantics

…uz, tucksaun)

This PR was merged into the 4.4 branch.

Discussion
----------

[Mailer] Move to the new DSN format

Documents symfony/symfony#33424

Cherry-picking #12258 that should have target 4.4 and also add a missing DSN update.

Commits
-------

a2c607b [Mailer] Move remaining DSN to new format
ca56502 Minor tweaks
be392bc Move to the new DSN format for Mailer