[META] Update / reconsider dependencies for Drupal 9

Problem/Motivation

Some dependencies need updating, some need to be reconsidered and removed or replaced.

Proposed resolution

The following are to be updated before Drupal 9.0.0 is released:

1. #3088369: Update Drupal 9 to Symfony 4.4-dev
2. #3041076: Update Drupal 9 to Twig 2
3. #2950132: Support PHPUnit 7 optionally in Drupal 8, while keeping support for ^6.5
4. #3091418: Update composer dependencies on 9.0.x (and followups)
5. #3104473: Use version of guzzlehttp/guzzle ^6.5.2
6. #3078671: Pin behat/mink and behat/mink-selenium2-driver to use resolvable release
7. #3105181: Remove egulias/EmailValidator
8. #3020296: [meta] Replace Symfony's classloader as it does not exist in Symfony 4
9. #3104265: Update Composer dependencies on Doctrine components in 9.0.x
10. #3104015: Replace ZendFramework/* dependencies by their Laminas equivalents
11. #3104354: Update Diactoros to 2.1
12. #3104353: Test Guzzle 7.0.0-beta1
13. #3094468: [plan] Update core JavaScript (and CSS) dependencies prior to 9.0.0-beta1
14. #3104265: Update Composer dependencies on Doctrine components in 9.0.x

There is also an ongoing issue at #2864037: [META] Update core PHP dependencies which is not Drupal-9-specific.

The following will happen later after Drupal 9.0.0 sometime in Drupal 9's lifetime:

1. #2966864: Add optional support for CKEditor 5 in D9 so we can remove CKE 4 from Drupal 10

The following are considered for removal or replacement: #3080837: [meta] PHP (and JS) dependencies to consider decoupling or removing

The following are already not used by Drupal 8 and will be removed in Drupal 9: #3032686: Remove references to unused packages in Drupal 9's vendor hardening

Remaining tasks

See above.

User interface changes

Likely none, depends on updates with UIs.

API changes

Depend on updated dependencies that Drupal is exposing as-is.

Data model changes

Depend on updates.