docs: add upgrading.md (#12477)

Co-authored-by: Marko <[email protected]>

UPGRADING.md

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+# Upgrading Cosmos SDK
+
+This guide provides instructions for upgrading to specific versions of Cosmos SDK.
+
+## v0.46
+
+### Client Changes
+
+### `x/gov` v1
+
+The `gov` module has been greatly improved. The previous API has been moved to `v1beta1` while the new implementation is called `v1`.
+
+In order to submit a proposal with `submit-proposal` you now need to pass a `proposal.json` file.
+You can still use the old way by using `submit-legacy-proposal`. This is not recommended.
+More information can be found in the gov module [client documentation](https://docs.cosmos.network/v0.46/modules/gov/07_client.html).
+
+### Keyring
+
+The keyring has been refactored in v0.46.
+
+* The `Unsafe*` interfaces have been removed from the keyring package. Please use interface casting if you wish to access those unsafe functions.
+* The keys' implementation has been refactored to be serialized as proto.
+* `keyring.NewInMemory` and `keyring.New` takes now a `codec.Codec`.
+* Take `keyring.Record` instead of `Info` as first argument in:
+        * `MkConsKeyOutput`
+        * `MkValKeyOutput`
+        * `MkAccKeyOutput`
+* Rename:
+        * `SavePubKey` to `SaveOfflineKey` and remove the `algo` argument.
+        * `NewMultiInfo`, `NewLedgerInfo`  to `NewLegacyMultiInfo`, `newLegacyLedgerInfo` respectively.
+        * `NewOfflineInfo` to `newLegacyOfflineInfo` and move it to `migration_test.go`.
+
+### Go API Changes
+
+The `replace google.golang.org/grpc` directive can be removed from the `go.mod`, it is no more required to block the version.
+
+A few packages that were deprecated in the previous version are now removed.
+
+For instance, the REST API, deprecated in v0.45, is now removed. If you have not migrated yet, please follow the [instructions](https://docs.cosmos.network/v0.45/migrations/rest.html).
+
+To improve clarity of the API, some renaming and improvements has been done:
+
+| Package   | Previous                           | Current                              |
+| --------- | ---------------------------------- | ------------------------------------ |
+| `simapp`  | `encodingConfig.Marshaler`         | `encodingConfig.Codec`               |
+| `simapp`  | `FundAccount`, `FundModuleAccount` | Functions moved to `x/bank/testutil` |
+| `types`   | `AccAddressFromHex`                | `AccAddressFromHexUnsafe`            |
+| `x/auth`  | `MempoolFeeDecorator`              | Use `DeductFeeDecorator` instead     |
+| `x/bank`  | `AddressFromBalancesStore`         | `AddressAndDenomFromBalancesStore`   |
+| `x/gov`   | `keeper.DeleteDeposits`            | `keeper.DeleteAndBurnDeposits`       |
+| `x/gov`   | `keeper.RefundDeposits`            | `keeper.RefundAndDeleteDeposits`     |
+| `x/{mod}` | package `legacy`                   | package `migrations`                 |
+
+For the exhaustive list of API renaming, please refer to the [CHANGELOG](https://github.com/cosmos/cosmos-sdk/blob/main/CHANGELOG.md).
+
+### new packages
+
+Additionally, new packages have been introduced in order to further split the codebase. Aliases are available for a new API breaking migration, but it is encouraged to migrate to this new packages:
+
+* `errors` should replace `types/errors` when registering errors or wrapping SDK errors.
+* `math` contains the `Int` or `Uint` types that are used in the SDK.
+
+### `x/authz`
+
+* `authz.NewMsgGrant` `expiration` is now a pointer. When `nil` is used, then no expiration will be set (grant won't expire).
+* `authz.NewGrant` takes a new argument: block time, to correctly validate expire time.
+
+### State Machine Changes
+
+#### PostHandler
+
+`postHandler` is like an `antehandler`, but is run _after_ the `runMsgs` execution. It is in the same store branch that `runMsgs`, meaning that both `runMsgs` and `postHandler`. This allows to run a custom logic after the execution of the messages.