Bump version to beta.7 and use variable for package version

- Update version from beta.6 to beta.7
- Change Directory.Packages.props to use $(Version) variable
- Single source of truth for version in source/Directory.Build.props

Author: StevenTCramer

Directory.Packages.props

@@ -1,6 +1,6 @@
 <Project>
   <ItemGroup>
-    <PackageVersion Include="TimeWarp.SourceGenerators" Version="1.0.0-beta.6" />
+    <PackageVersion Include="TimeWarp.SourceGenerators" Version="$(Version)" />
     <PackageVersion Include="FakeItEasy" Version="7.3.1" />
     <PackageVersion Include="Fixie" Version="3.2.0" />
     <PackageVersion Include="Fixie.TestAdapter" Version="3.3.0" />

source/Directory.Build.props

@@ -4,7 +4,7 @@
 
   <!-- Default package metadata (can be overridden in individual projects) -->
   <PropertyGroup Label="Package Metadata">
-    <Version>1.0.0-beta.6</Version>
+    <Version>1.0.0-beta.7</Version>
     <Authors>Steven T. Cramer</Authors>
     <Product>TimeWarp.SourceGenerators</Product>
     <PackageId>TimeWarp.SourceGenerators</PackageId>