build(deps): Bump dotnet/sdk in /EssentialCSharp.Web

Bumps dotnet/sdk from 8.0.403 to 9.0.100.

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

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

Author: dependabot[bot]

EssentialCSharp.Web/Dockerfile

@@ -5,7 +5,7 @@ WORKDIR /app
 EXPOSE 8080
 EXPOSE 8081
 
-FROM mcr.microsoft.com/dotnet/sdk:8.0.403 AS build
+FROM mcr.microsoft.com/dotnet/sdk:9.0.100 AS build
 RUN sh -c "$(curl -fsSL https://aka.ms/install-artifacts-credprovider.sh)"
 WORKDIR /src
 COPY . .