This PR was squashed before being merged into the 5.2-dev branch.

Discussion
----------

[Notifier] Change notifier recipient handling

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #35558 <!-- prefix each issue number with "Fix #", if any -->
| License       | MIT
| Doc PR        | tbd.

According to @wouterj's reasoning in #35558 I did the following to merge the `AdminRecipient` with the `Recipient` class:

- introduced a `EmailRecipientInterface` with `getEmail(): string` methods
- introduced a `RecipientInterface` that is extended by `EmailRecipientInterface` and `SmsRecipientInterface`
- changed `Recipient` class which now holds an email and a phone number property and implements the `EmailRecipientInterface` and the `SmsRecipientInterface`
- changed `NoRecipient` class which now implements the `RecipientInterface`
- removed the `AdminRecipient` and fixed all related type-hints
- introduced an `EmailRecipient` and `SmsRecipient`
- changed the type-hint of the `$recipient` argument in `NotifierInterface::send()`, `Notifier::getChannels()`, `ChannelInterface::notifiy()` and `ChannelInterface::supports()` to `RecipientInterface`
- changed the type hint of the `$recipient` argument in the `as*Message()` of the  `EmailNotificationInterface` and `SmsNotificationInterface` which now uses the introduced interfaces
- changed the type hint of the `$recipient` argument in the static `fromNotification()` factory methods in `EmailMessage` and `SmsMessage` which now uses the introduced interfaces
- changed `EmailChannel` to only support recipients which implement the `EmailRecipientInterface`
- changed `SmsChannel` only supports recipients which implement the `SmsRecipientInterface`

Commits
-------

588607b [Notifier] Change notifier recipient handling