Rename Unique Count and Percentiles

[figo-sh]

The names "Unique Count" and "Percentiles" are misleading, as they do not reflect the statistical nature of their underlying algorithms. I propose "Approximate Cardinality" (or "Approximate Unique Count") and "Approximate Percentiles".
It's a subtle difference, but can be of extreme importance, and isn't obvious unless you happen to actually read through the Elasticsearch manual.

[Bargs]

This is the case for the terms agg as well.

[epixa]

The crux of this issue is described in #8701, so I'm going to close this as a duplicate. Renaming them entirely is one possible option, though.