Deprecate cacheClass(), select cache storage internally

[TomasVotruba]

What

Moves the file-vs-memory cache storage decision out of user config and into CacheFactory, and deprecates the public cacheClass() API.

Why

cacheClass() exposed the CacheStorageInterface implementation as a public config knob, but there are only two implementations and only one is ever useful to end users:

• FileCacheStorage — the real, cross-run cache.
• MemoryCacheStorage — an in-process array, wiped when the PHP process exits.

A Rector CLI run processes every file once in a single process (hasFileChanged() loads before cacheFile() saves), so memory storage delivers zero cross-invocation benefit — the whole value of the cache is between runs. So cacheClass() was an implementation detail leaked as public API with no valid production use.

How

• CacheFactory now resolves the storage internally: an explicit cacheClass() still wins, otherwise file cache locally and in-memory cache in CI (where the ephemeral workspace makes writing a cache that is never re-read wasted IO). This is the exact policy that previously lived in config/config.php, just moved behind the factory — behaviour-preserving.
• RectorConfig::cacheClass() and the withCache(cacheClass:) argument are marked #[Deprecated]. They remain functional for the rare case that needs to force a specific storage — e.g. the e2e cache tests, which force FileCacheStorage so they can verify the persisted cache across two runs even on CI.
• Internal callers set the parameter directly instead of calling the deprecated method, so PHPStan's deprecation rules stay green.

No new environment variables or config surface. Split out from a combined branch; the FileCacheStorage::save() simplification is #8312.