[#767] Use `head~2` block in `branch` field of RPC operations

Description

Problem: We use the head block ID in the branch field of the run_operation, forge and preapply operations.

However, with the introduction of the ithaca protocol and the Tenderbake consensus algorithm, it is no longer safe to use the head block as the branch of those operations, because that block "is not necessarily final".

Solution: Use the head~2 block instead, as recommended by the docs: https://web.archive.org/web/20220305165609/https://tezos.gitlab.io/protocols/tenderbake.html

Related issue(s)

Resolves #767 (closed)

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