chore(deps): bump dotnet/sdk from 8.0 to 9.0

Bumps dotnet/sdk from 8.0 to 9.0.

---
updated-dependencies:
- dependency-name: dotnet/sdk
  dependency-version: '9.0'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Dockerfile

@@ -12,7 +12,7 @@ COPY ClientApp/ ./
 RUN npm run build
 
 # Stage 2: Build .NET application
-FROM mcr.microsoft.com/dotnet/sdk:8.0 AS backend-build
+FROM mcr.microsoft.com/dotnet/sdk:9.0 AS backend-build
 
 WORKDIR /src