[Backport 5.2] compaction: improve partition estimates when using garbage collected sstables

[mergify]

When a compaction strategy uses garbage collected sstables to track expired tombstones, do not use complete partition estimates for both the regular output sstables and the temporary sstables. Instead split the partition estimate between the two based on the droppable tombstone ratio estimate.

Fixes #18283

(cherry picked from commit 36b98d8)

Refs #18465

[mergify]

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[lkshminarayanan]

Replaced by #18659