[meta] Requirements for tagging Drupal 10.0.0-beta1

Must-haves prior to tagging 10.0.0-beta1

We can eventually file child metas for these, but a list is sufficient for now.

1. Update or decouple/remove PHP (Composer) dependencies.
   • Symfony:
     • Make Drupal 9 fully compatible with Symfony 5 as soon as possible: #3055180: [META] Make Drupal 9 work with Symfony 5 (along with Symfony 4)
     • Test with Symfony 6 as soon as a development version is available, and determine feasibility of requiring Symfony 6 for a longer Drupal 10 support lifetime.
   • Twig: Consider requiring Twig 3 #3126632: Require Twig 3
   • Guzzle: Either require Guzzle 7 or replace with a different implementation. #3104353: [TESTING PATCH ONLY, DO NOT COMMIT] Test Guzzle 7.0.0-beta1
   • Remove Symfony CMF dependency (or update it if we have to): #2917331: Decouple from Symfony CMF
   • Composer 2: #3126566: Composer 2 compatibility
   • Re-evaluate other backend dependencies in early to mid-2021.
   • (Other leftover issues from the Drupal 9 cycle to decouple/remove dependencies, etc.?)
2. Update or decouple/remove frontend dependencies.
   • CKEditor: Replace CKEditor 4 (either with CKEditor 5 or something else) #3091226: [META] Select the best modern editor for Drupal 9
   • jQuery UI: Replace remaining jQuery UI components, deprecate them in Drupal 9, and remove them in Drupal 10.
     • Finish deprecations: #3113400: Deprecate more jQuery UI library definitions
     • Autocomplete: #3076171: Provide an experimental module to replace jQuery UI autocomplete with awesomplete
     • jQuery cookie 3: #3118726: Evaluate/upgrade to js.cookie 3
     • Dialog: #2158943: Replace jQuery UI dialog with supported library or polyfill
   • jQuery: Upgrade to jQuery 4. https://github.com/jquery/jquery/milestone/7
   • Re-evaluate other frontend dependencies in early to mid-2021.
3. Remove all deprecated code, libraries, and BC layers.
4. Resolve critical 9.x -> 9.x upgrade path bugs.
5. Remove old upgrade paths based on the outcome of #2942096: [policy, no patch] Remove old update hooks prior to each major version (Drupal 10 and later).
6. Resolve any 10.0.x-only security or data integrity issues.
7. Define the Drupal 10 theme API.
   • At a minimum, create the Stable 10 base theme.

Should-haves prior to tagging 10.0.0-beta1

These become beta targets if they are not completed by the deadline, and may contribute to other adjustments to the schedule (TBD).

• (Possibly a must-have; TBD) Stabilize Starterkit, ensure a Drupal 10 version of the starter theme is generated, and move Classy to contrib.
• Update backend and frontend development dependencies.
• Other issues TBD.