build(backend): bump .net docker image to 7

Author: Liu233w

backend/dev.Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/sdk:6.0
+FROM mcr.microsoft.com/dotnet/sdk:7.0
 
 # needed in sln file
 RUN mkdir /build && echo '<?xml version="1.0" encoding="utf-8"?><Project ToolsVersion="15.0" Sdk="Microsoft.Docker.Sdk"></Project>' > /build/docker-compose.dcproj

backend/src/AcmStatisticsBackend.Web.Host/Dockerfile

@@ -4,7 +4,7 @@ FROM mcr.microsoft.com/dotnet/aspnet:7.0 AS base
 WORKDIR /app
 EXPOSE 80
 
-FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
+FROM mcr.microsoft.com/dotnet/sdk:7.0 AS build
 WORKDIR /src
 COPY ["src/AcmStatisticsBackend.Web.Host/AcmStatisticsBackend.Web.Host.csproj", "src/AcmStatisticsBackend.Web.Host/"]
 COPY ["src/AcmStatisticsBackend.Web.Core/AcmStatisticsBackend.Web.Core.csproj", "src/AcmStatisticsBackend.Web.Core/"]