Snap master to release/3.0 and rebrand preview 7 -> preview 8

[vatsan-madhavan]

• snap https://github.com/dotnet/wpf master -> release/3.0
  • Snap master -> release/3.0 for end of Preview 7 #1099
  • Snap master -> release/3.0 at the end of Preview 7 (PR no.2)  #1104 - 2nd snap to release
• rebrand https://github.com/dotnet/wpf master to preview 8
  • Rebrand master to Preview 8 #1102
• snap https://dev.azure.com/dnceng/internal/_git/dotnet-wpf-int master -> release/3.0
  • https://dev.azure.com/dnceng/internal/_git/dotnet-wpf-int/pullrequest/1852?_a=overview
• rebrand https://dev.azure.com/dnceng/internal/_git/dotnet-wpf-int master to preview 8
  • https://dev.azure.com/dnceng/internal/_git/dotnet-wpf-int/pullrequest/1853?_a=overview

/cc @wtgodbe

[vatsan-madhavan]

#1099 is opened to snap https://github.com/dotnet/wpf master -> release/3.0

[vatsan-madhavan]

#1102 rebrands master to preview 8