Update Azure Pipelines to install .NET 8.

Author: bgrainger

azure-pipelines.yml

@@ -23,7 +23,7 @@ resources:
  SA_PASSWORD: P@ssw0rd
 
 variables:
- DotNetCoreSdkVersion: '6.0.x'
+ DotNetCoreSdkVersion: '8.0.x'
 
 jobs:
  - job: Linux_MySQL