Update dcrinstall to download Bitcoin Core 0.21.2

Also update the decred versions while here for 1.7.0-rc1, although
these are usually set by the linker during release builds anyways.

cmd/dcrinstall/dcrinstall.go

@@ -130,23 +130,23 @@ var (
 	defaultLatestManifestURI = "https://raw.githubusercontent.com/decred/decred-release/master/latest"
 
 	defaultTuple                  = runtime.GOOS + "-" + runtime.GOARCH
-	defaultDecredManifestVersion  = "v1.6.0-rc3"
+	defaultDecredManifestVersion  = "v1.7.0-rc1"
 	defaultDecredManifestFilename = "decred-" + defaultDecredManifestVersion +
 		"-manifest.txt"
 	defaultDecredManifestURI = "https://github.com/decred/decred-binaries" +
 		"/releases/download/" + defaultDecredManifestVersion + "/" +
 		defaultDecredManifestFilename
 
 	// dcrdex
-	defaultDcrdexManifestVersion  = "v0.1.2"
+	defaultDcrdexManifestVersion  = "v0.4.0-rc1"
 	defaultDcrdexManifestFilename = "dexc-" + defaultDcrdexManifestVersion +
 		"-manifest.txt"
 	defaultDcrdexManifestURI = "https://github.com/decred/decred-binaries" +
 		"/releases/download/" + defaultDecredManifestVersion + "/" +
 		defaultDcrdexManifestFilename // Yes defaultDecredManifestVersion
 
 	// bitcoin
-	defaultBitcoinManifestVersion  = "0.20.1"
+	defaultBitcoinManifestVersion  = "0.21.2"
 	defaultBitcoinManifestFilename = "SHA256SUMS.asc"
 	defaultBitcoinManifestURI      = "https://bitcoincore.org/bin/" +
 		"bitcoin-core-" + defaultBitcoinManifestVersion + "/" +