refactor: Move sdk.Dec to math package (#12634)

## Description





---

### 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)

CHANGELOG.md

@@ -47,6 +47,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Improvements
 
+* [#12634](https://github.com/cosmos/cosmos-sdk/pull/12634) Move `sdk.Dec` to math package.
 * [#12596](https://github.com/cosmos/cosmos-sdk/pull/12596) Remove all imports of the non-existent gogo/protobuf v1.3.3 to ease downstream use and go workspaces. 
 * [#12589](https://github.com/cosmos/cosmos-sdk/pull/12589) Allow zero gas in simulation mode.
 * [#12576](https://github.com/cosmos/cosmos-sdk/pull/12576) Remove dependency on cosmos/keyring and upgrade to 99designs/keyring v1.2.1

go.mod

@@ -283,6 +283,7 @@ replace (
 	cosmossdk.io/api => ./api
 	cosmossdk.io/core => ./core
 	cosmossdk.io/depinject => ./depinject
+	cosmossdk.io/math => ./math
 	github.com/cosmos/cosmos-sdk/db => ./db
 	github.com/cosmos/cosmos-sdk/store/tools/ics23 => ./store/tools/ics23
 

go.sum

@@ -69,8 +69,6 @@ cloud.google.com/go/storage v1.14.0/go.mod h1:GrKmX003DSIwi9o29oFT7YDnHYwZoctc3f
 contrib.go.opencensus.io/exporter/stackdriver v0.13.4/go.mod h1:aXENhDJ1Y4lIg4EUaVTwzvYETVNZk10Pu26tevFKLUc=
 cosmossdk.io/errors v1.0.0-beta.7 h1:gypHW76pTQGVnHKo6QBkb4yFOJjC+sUGRc5Al3Odj1w=
 cosmossdk.io/errors v1.0.0-beta.7/go.mod h1:mz6FQMJRku4bY7aqS/Gwfcmr/ue91roMEKAmDUDpBfE=
-cosmossdk.io/math v1.0.0-beta.2 h1:17hSVc9ne1c31IaLDfjRojtN+y4Rd2N8H/6Fht2sBzw=
-cosmossdk.io/math v1.0.0-beta.2/go.mod h1:u/MXvf8wbUbCsAEyQSSYXXMsczAsFX48e2D6JI86T4o=
 dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
 filippo.io/edwards25519 v1.0.0-rc.1 h1:m0VOOB23frXZvAOK44usCgLWvtsxIoMCTBGJZlpmGfU=
 filippo.io/edwards25519 v1.0.0-rc.1/go.mod h1:N1IkdkCkiLB6tki+MYJoSx2JTY9NUlxZE7eHn5EwJns=

math/CHANGELOG.md

@@ -30,3 +30,5 @@ Ref: https://keepachangelog.com/en/1.0.0/
 # Changelog
 
 ## [Unreleased]
+
+* [#12634](https://github.com/cosmos/cosmos-sdk/pull/12634) Move `sdk.Dec` to math package, call it `LegacyDec`.