[6.x] Stache index cleanup

[jasonvarga]

Cleans up a few stache indexes that were storing whole objects or were never read.

Sites resolve to their handles

Site now implements QueryableValue, returning its handle, the same way Blueprint already does.

Without it, any value index that resolves a site (e.g. the forms-2 submission site index, which goes through the generic Value index) stores whole Site objects. Those indexes are persisted to the Laravel cache, and when cache.serializable_classes is restricted (newer Laravel skeletons default it to false), Site isn't on Statamic's allowlist. Any request that reads the cached index then gets __PHP_Incomplete_Class values and the query throws:

strtolower(): Argument #1 ($string) must be of type string, __PHP_Incomplete_Class given

Comparisons like where('site', Site::selected()) keep working since Site casts to its handle.

Entry and term site indexes

Entry::getQueryableValue('site') and LocalizedTerm::getQueryableValue('site') now return the locale directly, so their site store indexes use the regular value index. This avoids resolving a Site object per item, and an item whose site was removed still indexes its locale rather than null (same as before). The dedicated Indexes\Site and Indexes\Terms\Site classes are no longer used and have been marked deprecated.

Unused indexes removed

The submission form index and term taxonomy index are removed. where/whereIn on those columns are intercepted by SubmissionQueryBuilder / TermQueryBuilder to choose which child stores to read, so these eagerly built indexes (which held a Form / Taxonomy object per item) were never used for them. Any other query on those columns will still build the index lazily on first use.

Supersedes #15640.