Tidy up some of our Velocity features (#19605)

A few features were marked "always disabled" and then enabled in dev,
canary and preview. I simplified those to "always enabled" and disabled
in release instead. This required changes to Generate-FeatureStaging to
make it consider `WindowsInbox` a Release branding (which, honestly, it
always should have been.)

- Feature_DynamicSSHProfiles
- Feature_ShellCompletions
- Feature_SaveSnippet
- Feature_QuickFix

Feature_DisableWebSourceIcons was deprecated in #19143, but the XML file
never got the memo.

tools/Generate-FeatureStagingHeader.ps1

@@ -89,11 +89,11 @@ Function Resolve-FinalFeatureStage {
         [string]$Branding
     )
 
-    # RELEASE=DISABLED wins all checks
+    # RELEASE=DISABLED wins all checks (WindowsInbox counts as a type of Release)
     # Then, branch match by most-specific branch
     # Then, branding type (if no overriding branch match)
 
-    If ($Branding -Eq "Release" -And $Feature.DisabledReleaseToken) {
+    If ($Branding -In @("Release", "WindowsInbox") -And $Feature.DisabledReleaseToken) {
         [Stage]::AlwaysDisabled
         Return
     }