fix: update GH workflows and Docker image to .NET 6

Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build
+FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
 WORKDIR /app
 
 # Restore/fetch dependencies excluding app code to make use of caching
@@ -17,7 +17,7 @@ COPY . /app
 RUN dotnet build -c Release -o bin
 
 # Build runtime stage (doesn't include SDK)
-FROM mcr.microsoft.com/dotnet/aspnet:5.0
+FROM mcr.microsoft.com/dotnet/aspnet:6.0
 LABEL org.opencontainers.image.source = "https://github.com/xSke/PluralKit"
 
 WORKDIR /app