Update formula for PG-HyperLogLog small cardinality estimates

What does this MR do and why?

Based on the original HLL paper and referenced linear probabilistic counting paper, this should be m*log(m/V) instead of using the alpha(m) constant for small cardinality, as linear counting is used instead.

More detailed analysis in #348139 (comment 768958838)

This should also allow us to un-quarantine the test at #348139 (closed) (and hopefully make it less flaky at the current precision level?).

Screenshots or screen recordings

How to set up and validate locally

Helper scripts at https://gitlab.com/-/snippets/2220977 if needed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.