[TM-344] Remove Athens and Proxy from ManagedLedger

Description

Problem: Currently we maintain two different versions of Managed Ledger but since Babylon upgrade only one version (Babylon) is required.

Solution: Remove Athens-related functionality from ManagedLedger, simplify tests, move SaneParameter-related functionality from Proxy to ApprovableLedgerInterface so that other projects can use the provided interface.

Related issue(s)

https://issues.serokell.io/issue/TM-344 (This MR focuses on removing the Athens version of ManagedLedger, while TM-344 is broader. Other Athens-related features are to be removed in subsequent MRs)

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