fix(upgrade): bring the 2.0 upgrade up to date with Flarum 2.0.0

[imorland]

Fixes: no issue.

Changes proposed in this pull request:
upgrade 2.0 still targeted the RC period and PHPUnit 11. It no longer sets minimum-stability: beta. It drops the workflows' php_versions input, so the reusable workflow's default (PHP 8.3–8.5) applies, instead of writing a matrix with 8.2. It requires flarum-webpack-config ^3.0.6 (the first to load an extension's code-split chunks from its own build) and webpack ^5.104.1, in both the upgrade and the scaffold. The package.json update now reaches both dependencies and devDependencies; only one of them was updated before.

The PHPUnit step now targets PHPUnit 12, which reads no doc-comment metadata:

• every annotation with an attribute becomes one, on classes and methods;
• @dataProvider Class::method and @depends Class::method become *External attributes (the provider used to be silently dropped);
• assertions PHPUnit 10 renamed are renamed, and ->will($this->returnValue(…)) and its siblings become will*();
• what was removed without a replacement gets a @TODO.

Reviewers should focus on:

• The PHPUnit step's commit message changes, so an extension that ran the old step will run this one too. It is idempotent: a test runs it twice.
• Removed TestCase methods with common names (setLocale(), iniSet(), at(), expect*()) are only flagged on $this/self/static, because Flarum tests call $translator->setLocale().
• I ran the rewriter over all 180 PHPUnit 9 test files on framework's 1.x. Every output parses, all 840 @test and 46 @dataProvider annotations were converted, and no TODOs were needed.

Screenshot
N/A

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension? (N/A)
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (N/A)
• Frontend changes: tests are green (run yarn test in js/). (yarn test: 221 tests, plus Prettier, ESLint and tsc.)
• Frontend changes: tests have been added, or are not appropriate here. (31 new tests, written first and seen failing.)
• Backend changes: tests are green (run composer test). (N/A)
• Backend changes: tests have been added, or are not appropriate here. (N/A)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.

Required changes:

None.