chore: downgrade nmt to 0.12.0 (#1307)

Downgrade to NMT 0.12.0 while @distractedm1nd @liamsi investigate an
issue in 0.13.0

go.mod

@@ -3,7 +3,7 @@ module github.com/celestiaorg/celestia-app
 go 1.18
 
 require (
-	github.com/celestiaorg/nmt v0.13.0
+	github.com/celestiaorg/nmt v0.12.0
 	github.com/celestiaorg/quantum-gravity-bridge v1.3.0
 	github.com/ethereum/go-ethereum v1.10.26
 	github.com/gogo/protobuf v1.3.3

go.sum

@@ -192,8 +192,8 @@ github.com/celestiaorg/cosmos-sdk v1.6.0-sdk-v0.46.7 h1:wP4RRK9zbVBlQXqRqJXji09e
 github.com/celestiaorg/cosmos-sdk v1.6.0-sdk-v0.46.7/go.mod h1:MkOIxmKiICA4d8yPWo/590S4fqpaLXfocz0xn1fxBp4=
 github.com/celestiaorg/merkletree v0.0.0-20210714075610-a84dc3ddbbe4 h1:CJdIpo8n5MFP2MwK0gSRcOVlDlFdQJO1p+FqdxYzmvc=
 github.com/celestiaorg/merkletree v0.0.0-20210714075610-a84dc3ddbbe4/go.mod h1:fzuHnhzj1pUygGz+1ZkB3uQbEUL4htqCGJ4Qs2LwMZA=
-github.com/celestiaorg/nmt v0.13.0 h1:zhuwhvICMJhXy2lzaBydFBYztTuLzV5N3xAa9zTSiCw=
-github.com/celestiaorg/nmt v0.13.0/go.mod h1:b+pwd9cGTSSYLZnUIQSJl07pusJdFeEvCVsVfSRH9gA=
+github.com/celestiaorg/nmt v0.12.0 h1:6CmaMPri9FdSiytZP7yCrEq3ewebFiIEjlJhasrS6oQ=
+github.com/celestiaorg/nmt v0.12.0/go.mod h1:NN3W8EEoospv8EHCw50DDNWwPLpJkFHoEFiqCEcNCH4=
 github.com/celestiaorg/quantum-gravity-bridge v1.3.0 h1:9zPIp7w1FWfkPnn16y3S4FpFLnQtS7rm81CUVcHEts0=
 github.com/celestiaorg/quantum-gravity-bridge v1.3.0/go.mod h1:6WOajINTDEUXpSj5UZzod16UZ96ZVB/rFNKyM+Mt1gI=
 github.com/celestiaorg/rsmt2d v0.8.0 h1:ZUxTCELZCM9zMGKNF3cT+rUqMddXMeiuyleSJPZ3Wn4=