Replace multi-abi by allContracts and add a note regarding this change

[ggrieco-tob]

No description provided.

[montyly]

Note: there are multiple multi-abi left in this branch, including in the section titles

[ggrieco-tob]

This one is also ready for review @montyly

[montyly]

I noticed that its at least still present here: https://github.com/crytic/building-secure-contracts/blob/fix-use-allContracts/program-analysis/echidna/basic/common-testing-approaches.md

We should do a grep on the repo to ensure its up to date everywhere

[ggrieco-tob]

I did another merge with master and grep, I think fixed all the cases (except where it is explained that it changed in previous releases)