[META] Resolve Symfony 4 deprecations (Symfony 5 compatibility)

Problem/Motivation

In August 2019 Fabian Potencier announced at https://symfony.com/blog/symfony-maintenance-changes-for-standard-releases that

As of Symfony 5.0, we are changing the way we manage security issues for standard releases. A standard release is any minor version that is not a LTS release: so, versions X.0, X.1, x.2, and x.3.

For these standard releases, we will align the EOM (end of maintenance) date with the EOL (end of life) date. So, instead of having 14 months of security fixes, we will only have 8 months.

For instance, Symfony 4.3 EOM date is January 2020 and EOL date is July 2020. With the new rules, EOL would have been January 2020. Symfony 5.0 will be the first release to implement the change: EOM and EOL dates will be July 2020.

Therefore, to be able to support Drupal minor releases for 12 months, Drupal 9 choose to rely on Symfony 4.4. That said, Symfony 5 is already out and it may be possible to make Drupal 9 both Symfony 4.4 and Symfony 5 compatible. This issue is to explore to do that as much as possible. This will be very useful in the transition to Drupal 10 as well, which is planned to be released on Symfony 5.4 Long Term Support.

See #2976394: Allow Symfony 4.4 to be installed in Drupal 8 for prior art with Symfony 4 on Drupal 8.

Proposed resolution

See children issues.

Remaining tasks

Three issues for EventDispatcher:

#3055198: [Symfony 5] Symfony/Component/EventDispatcher/Event is deprecated in Symfony 4.3 use Symfony/Contracts/EventDispatcher/Event instead

#3055194: [Symfony 5] The signature of the "Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher::dispatch()" method should be updated to "dispatch($event, string $eventName = null)", not doing so is deprecated since Symfony 4.3.

(follow-up, not a hard blocker here:) #3153803: [PP-2] Find a way to notify people about Symfony Event class changes

One issue for MimeTypes:

#3055193: [Symfony 5] The "Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesser" class is deprecated since Symfony 4.3, use "Symfony\Component\Mime\MimeTypes" instead.

User interface changes

None.

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD