chore: update gov router note (#12508)

## Description

Clarifies the gov router usage in app.go


---

### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

simapp/app_legacy.go

@@ -298,7 +298,10 @@ func NewSimApp(
 	*/
 	app.GroupKeeper = groupkeeper.NewKeeper(keys[group.StoreKey], appCodec, app.MsgServiceRouter(), app.AccountKeeper, groupConfig)
 
-	// register the proposal types
+	// Register the proposal types
+	// Deprecated: Avoid adding new handlers, instead use the new proposal flow
+	// by granting the governance module the right to execute the message.
+	// See: https://github.com/cosmos/cosmos-sdk/blob/release/v0.46.x/x/gov/spec/01_concepts.md#proposal-messages
 	govRouter := govv1beta1.NewRouter()
 	govRouter.AddRoute(govtypes.RouterKey, govv1beta1.ProposalHandler).
 		AddRoute(paramproposal.RouterKey, params.NewParamChangeProposalHandler(app.ParamsKeeper)).