Fix the velocity script to actually support canary (microsoft#16810)

Welp, would you look at that? We never actually supported "canary"
feature settings. Canary's been defaulting to the "Dev" config since
inception.

There's a couple things that are supposed to only be on in Dev and not
Canary. They clearly haven't mattered, but better safe than sorry!

build/rules/GenerateFeatureFlags.proj

@@ -30,6 +30,7 @@
     <IntermediateOutputPath>$(SolutionDir)obj\$(Configuration)\GenerateFeatureFlags\</IntermediateOutputPath>
     <OpenConsoleCommonOutDir>$(SolutionDir)bin\$(Configuration)\</OpenConsoleCommonOutDir>
 
+    <_WTBrandingName Condition="'$(WindowsTerminalBranding)'=='Canary'">Canary</_WTBrandingName>
     <_WTBrandingName Condition="'$(WindowsTerminalBranding)'=='Preview'">Preview</_WTBrandingName>
     <_WTBrandingName Condition="'$(WindowsTerminalBranding)'=='Release'">Release</_WTBrandingName>
     <_WTBrandingName Condition="'$(_WTBrandingName)'==''">Dev</_WTBrandingName>