CET: Remove Measure.Upstream_Flag

The Upstream flag for deemed claims is no longer populated in eTRM measure permutations. Beginning in 2026, upstream claims can instead be identified using the delivery type (e.g., “Up‑Manuf”). However, the Upstream flag is still submitted as part of the measure input, so removing it would affect both the claims specification and the CET. To minimize impact, this change should be bundled with the next CET update (i.e., included in the 2027 ADP).