`:retry_timeout` and `:retry` got removed because reconnecting on failure is
something the new pool intentionally doesn't do.
`:max_overflow` had to go in favor of `:max_waiting`, I didn't reuse the key because
the settings are very different in their behaviour.
`:checkin_timeout` got removed in favor of `:connection_acquisition_wait`,
I didn't reuse the key because the settings are somewhat different.
I didn't do any migrations/deprecation warnings/changelog entries because
these settings were never in stable.
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