release-23.2: allocator: lower default IO overload threshold

[blathers-crl]

Backport 1/1 commits from #113667 on behalf of @kvoli.

/cc @cockroachdb/release

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The IO overload threshold determines whether a store will be excluded as a target for rebalancing replicas, or leases onto it. The previous thresholds were 0.8 for replica rebalancing
(kv.allocator.replica_io_overload_threshold) and 0.5 for lease transfers (kv.allocator.lease_io_overload_threshold).

The previous settings were selected when Admission Control (AC) attempted to stabilize IO overload at a threshold of 1.0. AC will now typically stabilize a store to a threshold of 0.5.

Lower the thresholds to 0.4 for replica rebalancing and 0.3 for lease transfers. Note that a store needs to both exceed the absolute threshold mentioned above, as well as be greater than +10% * mean of equivalent rebalancing candidates.

Resolves: #112497
Release note: None

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Release justification: Brings thresholds in line with AC throttling in 23.2. Without this update, the default IO overload value will rarely/never be hit to more aggressive AC throttling.

[blathers-crl]

Thanks for opening a backport.

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @kvoli)

[kvoli]

Rebased on release-23.2 TYFTRS!

[kvoli]

TestSQLStatsIdleLatencies failed on extended stress race. Failure looks unrelated, merging.