chore: api sub-module vanity URL (#11941)

## Description

ref: #11906

- Update `api` module path (`api/go.mod`)
- Update relevant `buf.gen.*.yaml` configurations
- Update root `go.mod` via replace directive (tag will come after this PR is merged)

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
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

*All items are required. Please add a note if the item is not applicable and please add
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)

testutil/testdata/buf.gen.pulsar.yaml

@@ -8,7 +8,7 @@ managed:
       - buf.build/cosmos/gogo-proto
       - buf.build/cosmos/cosmos-proto
     override:
-      buf.build/cosmos/cosmos-sdk: github.com/cosmos/cosmos-sdk/api
+      buf.build/cosmos/cosmos-sdk: cosmossdk.io/api
 plugins:
   - name: go-pulsar
     out: ../testdata_pulsar

testutil/testdata_pulsar/unknonwnproto.pulsar.go

@@ -2,10 +2,10 @@
 package testdata_pulsar
 
 import (
+	v1beta1 "cosmossdk.io/api/cosmos/tx/v1beta1"
 	binary "encoding/binary"
 	fmt "fmt"
 	runtime "github.com/cosmos/cosmos-proto/runtime"
-	v1beta1 "github.com/cosmos/cosmos-sdk/api/cosmos/tx/v1beta1"
 	_ "github.com/gogo/protobuf/gogoproto"
 	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
 	protoiface "google.golang.org/protobuf/runtime/protoiface"