[TM-344] Simplify user-defined upgrades

Description

Problem:

Currently, user-defined upgrades contain various workarounds to solve issues that do not exist anymore in Tezos mainnet, namely:

1. It used to be impossible to call some contract without knowing its full parameter type, hence:

• The documentation proposes two fairly complicated ways to call V2 from V1 (lambdas and a proxy contract)
• The implementation uses the lambda approach instead of addressing V2 via entrypoints

2. It used to be non-trivial to place big map arbitrarily in storage, thus he implementation used a Map instead of a BigMap to store balances.

Since it is no longer required to know the full parameter type to call an entrypoint, and the placement restrictions on big maps have been lifted, it is natural to simplify both the spec and the implementation of user-defined upgrades.

Solution:

1. Use explicit entrypoints instead of workarounds both in the docs and in the implementation.
2. Replace a Map with a BigMap

Related issue(s)

Resolves https://issues.serokell.io/issue/TM-344

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