Centralize app version info in project file for maintainability (#2055)

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## Description
<!--- Describe your changes in detail -->

This PR centralizes the app version management by introducing a
`Version` property in the app project file.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here. -->

Currently, we are manually updating these files to bump the app version,
which is error-prone and easy to miss.

- Package.appxmanifest
- Package.Dev.appxmanifest
- AssemblyInfo.cs

Closes #2054.

## How Has This Been Tested?
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to -->
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Ran the app and confirmed the changes.

## Screenshots (if appropriate):

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)

Author: AndrewKeepCoding

WinUIGallery/Package.Dev.appxmanifest

@@ -1,11 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Package
-    xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10"
-    xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
-    xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10"
-    xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3"
-    xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
-    IgnorableNamespaces="uap mp uap3">
+<Package xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10" xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest" xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10" xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3" xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities" IgnorableNamespaces="uap mp uap3">
   <!--Note: package version and assembly version must be kept in sync-->
   <Identity Name="Microsoft.WinUI3ControlsGallery.Debug" Publisher="CN=Microsoft Corporation, O=Microsoft Corporation, L=Redmond, S=Washington, C=US" Version="2.7.0.0" />
   <mp:PhoneIdentity PhoneProductId="863667e0-667a-4bb4-ac52-c59656c7333a" PhonePublisherId="00000000-0000-0000-0000-000000000000" />
@@ -16,7 +10,7 @@
     <uap:SupportedUsers>multiple</uap:SupportedUsers>
   </Properties>
   <Dependencies>
-      <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.17763.0" MaxVersionTested="10.0.19041.0" />
+    <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.17763.0" MaxVersionTested="10.0.19041.0" />
   </Dependencies>
   <Resources>
     <Resource Language="x-generate" />

WinUIGallery/Package.appxmanifest

@@ -1,11 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Package
-    xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10"
-    xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
-    xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10"
-    xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3"
-    xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
-    IgnorableNamespaces="uap mp uap3">
+<Package xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10" xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest" xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10" xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3" xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities" IgnorableNamespaces="uap mp uap3">
   <!--Note: package version and assembly version must be kept in sync-->
   <Identity Name="Microsoft.WinUI3ControlsGallery" Publisher="CN=Microsoft Corporation, O=Microsoft Corporation, L=Redmond, S=Washington, C=US" Version="2.7.0.0" />
   <mp:PhoneIdentity PhoneProductId="863667e0-667a-4bb4-ac52-c59656c7333a" PhonePublisherId="00000000-0000-0000-0000-000000000000" />
@@ -16,7 +10,7 @@
     <uap:SupportedUsers>multiple</uap:SupportedUsers>
   </Properties>
   <Dependencies>
-      <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.17763.0" MaxVersionTested="10.0.19041.0" />
+    <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.17763.0" MaxVersionTested="10.0.19041.0" />
   </Dependencies>
   <Resources>
     <Resource Language="x-generate" />
@@ -31,7 +25,7 @@
             <uap:ShowOn Tile="square310x310Logo" />
           </uap:ShowNameOnTiles>
         </uap:DefaultTile>
-        <uap:SplashScreen Image="Assets\Tiles\SplashScreen.png"/>
+        <uap:SplashScreen Image="Assets\Tiles\SplashScreen.png" />
       </uap:VisualElements>
       <Extensions>
         <uap3:Extension Category="windows.appUriHandler">