[Validator] Change default mode of `Assert\Email` to "strict", for consistency with Mailer

[ThomasLandauer]

Description
Since Symfony Mailer is using egulias/email-validator to validate email addresses, I'm suggesting to make this the default for Assert\Email.

Why? When using a framework to let users enter their email address, my minimum expectation would be that the very same framework is able to send an email there ;-) In other words: Symfony needs to stay consistent.

See #41850 for an example I encountered in production - but I'm sure there are better ones ;-)

[stof]

Changing the default in the validator component would break things for projects that don't have egulias/email-validator as dependency.
However, there is already a config option to configure the default strictness of the email validation globally in the project (for cases where the mapping does not configure it explicitly). A way to solve that would be to have a smart default in FrameworkBundle for that config based on egulias/email-validator being installed or no.

[ThomasLandauer]

After some more thinking about it, I'm now suggesting:

• Drop loose mode completely. @javiereguiluz and me are having an ongoing discussion whether loose is "too simple" or just "very simple" ([Validator] Adding missing link target symfony-docs#15853 and Shortening "mode" symfony-docs#15856) My bottom line: If you really think that loose is useless, then drop it! If somebody wants to accept virtually anything in their email field, they can just omit the assertion ;-)
• Make strict the default if egulias/email-validator is present, and html5 otherwise - just as @stof suggested above.

[javiereguiluz]

My proposal:

• Remove loose mode
• Make html5 mode the default one
• Don't make egulias/email-validator a hard dependency

Why?

• html5 is like loose done right and it doesn't require any dependency
• It's OK to require egulias/email-validator when using the Mailer component (it's 100% related to it) but it's not OK doing the same in the Validator component (where using the Email constraint is 100% optional, so you may never actually use it)

[fabpot]

I like your plan @javiereguiluz
Ideally, we would deprecate the loose mode in 5.4 to be able to remove it in 6.0

Who wants to create the PR on 5.4?