Partition stream by (wiki_id, page_id)
Partition page_source by (wiki_id, page_id).
This change is mostly cosmetic (not perf related) and syncs mw-page-content-change-enrich partition scheme
with the key pair declared by eventgate for this stream.
Tested with live traffic on eqiad, with no regression wrt the previous partitioning scheme (by wiki_id). Metrics checked:
- watermark lag.
- avg time for async operations.
- ps util memory.
Bug: T338169