Bump dotnet/aspnet from 8.0-alpine to 9.0-alpine in /src/backend/WebApi

Bumps dotnet/aspnet from 8.0-alpine to 9.0-alpine.

---
updated-dependencies:
- dependency-name: dotnet/aspnet
  dependency-type: direct:production
...

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

src/backend/WebApi/Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/aspnet:8.0-alpine AS base
+FROM mcr.microsoft.com/dotnet/aspnet:9.0-alpine AS base
 WORKDIR /app
 RUN apk add --no-cache icu-libs
 ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=false