[release-v2.1] main: Use backported mixing updates.

This updates the 2.1 release branch to use the latest version of the
mixing module which includes various optimizations, orphan source
tracking, proactive orphan eviction, tighter standardness limits on
mixing message sizes, and stricter rejection of malformed mixing pair
requests.

In particular, the following updated module version is used:

- github.com/decred/dcrd/mixing@v0.7.0

Author: davecgh

go.mod

@@ -26,7 +26,7 @@ require (
 	github.com/decred/dcrd/dcrutil/v4 v4.0.3
 	github.com/decred/dcrd/gcs/v4 v4.1.1
 	github.com/decred/dcrd/math/uint256 v1.0.2
-	github.com/decred/dcrd/mixing v0.6.1
+	github.com/decred/dcrd/mixing v0.7.0
 	github.com/decred/dcrd/peer/v3 v3.2.0
 	github.com/decred/dcrd/rpc/jsonrpc/types/v4 v4.4.0
 	github.com/decred/dcrd/rpcclient/v8 v8.1.0

go.sum

@@ -56,8 +56,8 @@ github.com/decred/dcrd/hdkeychain/v3 v3.1.3 h1:Kn2wfj5cOR6pQO/WrYOMT1KK12IgWFEeQ
 github.com/decred/dcrd/hdkeychain/v3 v3.1.3/go.mod h1:mDAuGaH6InRD+hKVeVJsjLD/ih1mD3aCKURNHS8Tq2s=
 github.com/decred/dcrd/math/uint256 v1.0.2 h1:o8peafL5QmuXGTergI3YDpDU0eq5Z0pQi88B8ym4PRA=
 github.com/decred/dcrd/math/uint256 v1.0.2/go.mod h1:7M/y9wJJvlyNG/f/X6mxxhxo9dgloZHFiOfbiscl75A=
-github.com/decred/dcrd/mixing v0.6.1 h1:nysHraShVbgKIrqpa04364YTchmVtqgaphTNWqs7nHc=
-github.com/decred/dcrd/mixing v0.6.1/go.mod h1:M+Ao9h49usdSEsC0N6kMafijwgYh7E57RTKh8cCw02o=
+github.com/decred/dcrd/mixing v0.7.0 h1:rKs5/6E8szo561kKp54iXdcC7GlQBIE9IP3utgKxjAM=
+github.com/decred/dcrd/mixing v0.7.0/go.mod h1:UE7+nPvXA6C7GdIFXJmz0ksmxykwn88zzbQFwAopahE=
 github.com/decred/dcrd/peer/v3 v3.2.0 h1:6PxoGcsyjV3me/WLjIZdmWs4Pma5jYGi40CmzNCzeNY=
 github.com/decred/dcrd/peer/v3 v3.2.0/go.mod h1:OUFvMboEQvnq4V8CRw2RUn/fFls0rpVuDN0QT0zq8RA=
 github.com/decred/dcrd/rpc/jsonrpc/types/v4 v4.4.0 h1:BBVaYemabsFsaqNVlCmacoZlSsLDqwHYIs8ty6fg59Q=