[META] Beta targets following Drupal 9.0.0-beta1

Problem/Motivation

In #3079680: [META] Requirements for tagging Drupal 9.0.0-beta1, we define must-haves and should-haves for tagging beta1. Some should-haves may cause Drupal 9.0.0-beta1 to be deferred to one of the predefined fallback windows, but we may allow some to be completed during the beta and release candidate phases.

Proposed resolution

This is a stub issue for tracking critical beta target tasks that should happen during the Drupal 9.0 beta phase following 9.0.0-beta1. At a minimum, it will include any should-haves from #3079680: [META] Requirements for tagging Drupal 9.0.0-beta1 not completed by 9.0.0-beta1.

It may include other tasks for core, Drupal.org, documentation, ecosystem tools, etc.

Beta-sensitive bugs

Critical regressions due to recent commits, bugs affecting the upgrade and migration paths to 8.9 and 9.0

1. #3120910: Sites with missing schema information can't update to 8.8.3+ - attempts to run all historical updates
2. #3100712: Drupal 8.7.10 to 8.8.0 update fails if views have invalid configuration
3. #3118625: Some older revisions do not get revision_translation_affected marked by migration when Workbench Moderation is used
4. #3126658: Support enclosing reserved words with brackets.

Must-have (*)

(*) We won't actually delay the scheduled release once we've locked in a beta scenario, but we might delay 9.0.0-rc1 for them.

1. DONE: Dependency updates:
   #3127674: Update dependencies for Drupal 9.0
2. DONE: Email contributed project authors asking them to test the beta and port their modules.
3. DONE: Launch beta testing program:
   #3118372: [meta] 8.9.0 beta testing and #3118366: [meta] 9.0.0 beta testing
4. In-core update/upgrade documentation:
   #3090183: UPGRADE.txt only mentions migrating from Drupal 6 and 7, UPDATE.txt only mentions 8-8 updates
5. Upgrade path regression:
   #3120910: Sites with missing schema information can't update to 8.8.3+ - attempts to run all historical updates
6. Extension validation regression"
   #3123537: /admin/reports/status should not WSOD if it finds an extension missing core_version_requirement in its info.yml file
7. Dependency support:
   • DONE: #3107918: Require Node.js 12
   • DONE: #3118726: Upgrade to js.cookie 3
   • #3126566: Allow Drupal to work with Composer 2

Should-have

1. More contrib -> core migration paths:
   • #2456259: [META] Drupal 7 to Drupal 8 Migration path
   • #2208401: [META] Remaining multilingual migation paths
   • #3076447: Migrate D7 entity translation revision translations to D8
2. Forward compatibility shims for third-party libraries: #3126695: [D8 only] Add forwards-compatibility shim for assertEqualsCanonicalizing() in phpunit 6&7

Could-have

Needs triage

• Beta target issue queue
• #3131950: UX review of the finished, semver-compliant update status report for how it presents the Drupal 8.9 and 9.0 updates for users on various versions
• Other tasks TBD.