[5.x] Modernize Google API Client for Drupal 11 and 12

Problem/Motivation

The 4.x branch supports Drupal 8 through 11 and retains a very old PHP compatibility baseline. That broad compatibility prevents us from adopting current Drupal APIs and modern PHP practices cleanly. Drupal 11.3 reports deprecations, including the legacy requirements hook warning tracked in #3609690, and the module also has PHPStan findings that should be resolved.

The 5.x major release will provide a clean compatibility boundary. It will drop Drupal 8, 9, and 10, support Drupal 11 and 12 properly, and allow general improvements that are not suitable for the 4.x maintenance branch.

Goals

• Support Drupal 11.3 or newer and Drupal 12.
• Require PHP 8.3 or newer. Drupal 12 installations will require PHP 8.5 through Drupal core.
• Resolve Drupal 11 and 12 deprecations.
• Establish and pass an agreed PHPStan level.
• Add automated coverage for authentication, token refresh, invalid grants, revocation, and service-account clients.
• Modernize dependency injection, type declarations, and service definitions where doing so improves maintainability.
• Preserve existing configuration and provide a documented upgrade path from 4.x.

Proposed resolution

Create a 5.x branch from the latest 8.x-4.x release with the following baseline:

• core_version_requirement: ^11.3 || ^12
• php: >=8.3
• Supported releases of google/apiclient and google/apiclient-services

Keep 8.x-4.x as the maintenance branch for existing sites. Limit it to important bug and security fixes while 5.x is developed.

Planned child work

• Update core, PHP, Composer, and Google client dependency requirements.
• Replace deprecated procedural hooks with supported Drupal 11 and 12 implementations.
• Resolve PHPStan findings and define the enforced analysis level.
• Modernize services and dependency injection.
• Review public interfaces and add safe PHP type declarations.
• Consolidate and test token validation, refresh, revocation, and error handling.
• Add a Drupal 11 and 12 test matrix.
• Document breaking changes and the 4.x to 5.x upgrade path.

Non-goals

• Do not change configuration identifiers or discard stored client configuration without a migration path.
• Do not combine all modernization work into one large merge request. Each child issue should remain focused and independently reviewable.

Definition of done

• Drupal 11 and 12 compatibility checks pass.
• Supported PHP versions pass automated checks.
• No known Drupal deprecation warnings remain.
• Authentication and token lifecycle tests pass.
• The upgrade path and breaking changes are documented.
• A 5.0.0-alpha1 release is ready for contributed-module testing.