PR-Validation build badge update (microsoft#3524)

- Move our CI PR-Validation build to the YAML-based build definition.
- Retire the old VSTS-style PR-Validation build.

CONTRIBUTING.md

@@ -7,7 +7,7 @@
 
 | macOS/Windows CI | Linux CI | Rolling CI (macOS/Windows) | Code Coverage |
 |-|-|-|-|
-|[![Build status](https://vscode-python.visualstudio.com/VSCode-Python/_apis/build/status/VSCode-Python-CI)](https://vscode-python.visualstudio.com/VSCode-Python/_build/latest?definitionId=4) | [![Build Status (Travis)](https://travis-ci.org/Microsoft/vscode-python.svg?branch=master)](https://travis-ci.org/Microsoft/vscode-python/branches) | [![Build status](https://vscode-python.visualstudio.com/VSCode-Python/_apis/build/status/VSCode-Python-Rolling-CI)](https://vscode-python.visualstudio.com/VSCode-Python/_build/latest?definitionId=9) | [![codecov](https://codecov.io/gh/Microsoft/vscode-python/branch/master/graph/badge.svg)](https://codecov.io/gh/Microsoft/vscode-python)|
+|[![Build Status](https://vscode-python.visualstudio.com/VSCode-Python/_apis/build/status/vscode-python-ci-pr_validation)](https://vscode-python.visualstudio.com/VSCode-Python/_build/latest?definitionId=18) | [![Build Status (Travis)](https://travis-ci.org/Microsoft/vscode-python.svg?branch=master)](https://travis-ci.org/Microsoft/vscode-python/branches) | [![Build status](https://vscode-python.visualstudio.com/VSCode-Python/_apis/build/status/VSCode-Python-Rolling-CI)](https://vscode-python.visualstudio.com/VSCode-Python/_build/latest?definitionId=9) | [![codecov](https://codecov.io/gh/Microsoft/vscode-python/branch/master/graph/badge.svg)](https://codecov.io/gh/Microsoft/vscode-python)|
 
 [[Development build](https://pvsc.blob.core.windows.net/extension-builds/ms-python-insiders.vsix)]