fix smoke test: use mcr_channel key and stable-25.0 for upgrade

LAUNCHPAD map has no 'mcr_version' key — it uses 'mcr_channel'.
The type assertion on a nil map value caused a panic at line 172.

Use the correct key and replace the channel string with 'stable-25.0'
(floating channel, picks up latest patch) for the upgrade step.

Author: james-nesbitt

test/smoke/smoke_test.go

@@ -169,7 +169,7 @@ func TestSupportedMatrixCluster(t *testing.T) {
 	assert.NoError(t, err)
 
 	// Replace the version values for MCR,MKE,MSR in the mkeClusterConfig for an upgrade
-	mkeClusterConfig = strings.ReplaceAll(mkeClusterConfig, LAUNCHPAD["mcr_version"].(string), "25.0.13")
+	mkeClusterConfig = strings.ReplaceAll(mkeClusterConfig, LAUNCHPAD["mcr_channel"].(string), "stable-25.0")
 	mkeClusterConfig = strings.ReplaceAll(mkeClusterConfig, LAUNCHPAD["mke_version"].(string), "3.8.8")
 	mkeClusterConfig = strings.ReplaceAll(mkeClusterConfig, LAUNCHPAD["msr_version"].(string), "2.9.28")