fix(upgrade): actually modernize phpunit XML configs

[imorland]

The bug — the XML rewrite has never run

Found while modernizing fof/synopsis's configs by hand (FriendsOfFlarum/synopsis#7) and asking why the upgrade tool hadn't already done it. Two independent defects, either fatal alone:

replacements(file: string): Replacement[] {
    if (!file.endsWith('.php')) return [];   // ← XML files never get replacements
    return [
      ...,
      (file, code) => {
        if (!file.endsWith('.xml')) return null;   // ← unreachable
        return {
          // ← String.replace with regex syntax in a plain string: literal match, hits nothing
          updated: code.replace('xsi:noNamespaceSchemaLocation="([^"]+)"', '...'),
        };
      },
    ];
}

So every extension run through upgrade:2.0 kept its PHPUnit 9.3-era config — which PHPUnit 12 tolerates but flags as a deprecation on every run, hides query-guard warnings behind summary counts, and pins the schema to a URL that no longer matches the installed phpunit.

The fix

modernizePhpunitXml() in its own dependency-free module, wired through a restructured dispatch (.php and phpunit*.xml each get their own path; root-level phpunit.xml added to targets). The transform converts to the shape this CLI scaffolds:

• schema from the vendored phpunit, depth-aware (tests/… → ../vendor/…, root → vendor/…)
• removed-in-10 attributes dropped; backupStaticAttributes → backupStaticProperties keeping its value
• cacheDirectory + displayDetailsOnTestsThatTriggerWarnings inserted in scaffold order
• <coverage> → <source> (include list preserved); <listeners> removed
• testsuites and per-file settings untouched; idempotent on already-modern configs

Verification

• 9 new jest tests written RED first (fixtures are synopsis's actual old configs): schema paths at both depths, attribute removal/rename, insertions, coverage→source, listeners removal, preservation, idempotency
• Full suite 190/190, Prettier clean
• End-to-end: the built binary run against a fixture extension carrying the old config produced byte-for-byte the scaffolded modern shape