Bump the podspec version and changelog version for release patch (#6352)

We'll check for greenness in the patch.

FirebaseAppDistribution.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'FirebaseAppDistribution'
-  s.version          = '0.9.0'
+  s.version          = '0.9.1'
   s.summary          = 'App Distribution for Firebase iOS SDK.'
 
   s.description      = <<-DESC

FirebaseAppDistribution/CHANGELOG.md

@@ -1,4 +1,4 @@
-# Unreleased
+# v0.9.1
 - [changed] Updated header comments (#6321).
 - [fixed] Bug for customers with restricted API keys unable to fetch releases (#6346).