:fix: set hikari configuration pool to fixed size (more performant)

[riccardomodanese]

Brief description of the PR. Fixes Hikari pool configuration to achieve better performances (fixed size). Should we remove the 2 no more used options like SystemSettingKey.DB_POOL_IDLE_TIMEOUT and SystemSettingKey.DB_POOL_SIZE_MIN and their default values?

Related Issue none

Description of the solution adopted Remove

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