chore(deps): update dotnet monorepo to v9 (major) (#72)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
|
[mcr.microsoft.com/dotnet/aspnet](https://redirect.github.com/dotnet/aspnetcore)
| final | major | `8.0` -> `9.0` |
| [mcr.microsoft.com/dotnet/sdk](https://redirect.github.com/dotnet/sdk)
| stage | major | `8.0` -> `9.0` |

---

### Configuration

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rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about these
updates again.

---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
this box

---

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Dockerfile

@@ -1,5 +1,5 @@
 ### Build
-FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
+FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
 
 WORKDIR /app
 
@@ -17,7 +17,7 @@ RUN dotnet publish \
   MumbleApi
 
 ### Deploy
-FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS final
+FROM mcr.microsoft.com/dotnet/aspnet:9.0 AS final
 
 ARG BUILD_VERSION=unknown
 ARG COMMIT_SHA=unknown