xds/clusterimpl: fix uint32 overflow computing EDS drop ratio (#9257)

handleClusterConfigLocked turns each EDS drop_overload into a
per-million rate with `Numerator * million / Denominator` in uint32. The
numerator comes from the control plane and reaches a million for a 100%
drop, so with the million denominator the product overflows a uint32
once the numerator hits 4295 (a 0.43% drop), and the rate then collapses
toward zero: a 50% drop computes to 0.18% and a 100% drop to 0.36%. A
fraction above 100% instead lands `RequestsPerMillion` above a million,
which underflows `million - RequestsPerMillion` in newDropper.

Move the computation into `dropRequestsPerMillion`, which multiplies in
uint64 and caps the ratio at a million. Keeping the clamp next to the
arithmetic that builds DropConfig leaves the raw numerator and
denominator on the parsed resource and applies drops at the configured
rate.

RELEASE NOTES:
* xds: fix xDS EDS drop policies being applied at a much lower rate than
configured due to an integer overflow