Spin cosmossdk.io/x/auth back in Cosmos SDK

[julienrbrt]

The Cosmos SDK depends heavily on x/auth, for v0.52 the team has decided not to extract auth in its own go.mod (as auth depends on the SDK and the SDK on auth).
Instead, cosmossdk.io/x/auth will exist again as github.com/cosmos/cosmos-sdk/x/auth.
This will be the only not extracted module (-- maybe bring back params as well, as it is deprecated?).
The Cosmos SDK has for goal to allow a chain to only start with x/accounts, an alternative to x/auth.

Tasks

• Revert this for x/auth: https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#gomod
• Remove cosmossdk.io/x/auth from cosmos sdk main README.md
• Add disclaimer in UPGRADING.md about the reason