Bump .NET SDK to 5.0.100, bump C# to 9.0

BizHawk.Tests executable needs to run in CI, but the 5.0 SDK image can't run
Core 3.1 apps somehow, so I bumped that too

.gitlab-ci.yml

@@ -13,7 +13,7 @@ build:
     name: "$CI_HAWK_TMPARTIFACT_NAME"
     paths:
       - output
-  image: mcr.microsoft.com/dotnet/core/sdk:3.1
+  image: mcr.microsoft.com/dotnet/sdk:5.0
   script:
     - if [ "$CI_COMMIT_REF_SLUG" == "release" ]; then Dist/UpdateVersionInfoForRelease.sh; fi
     - Dist/BuildRelease.sh
@@ -21,7 +21,7 @@ build:
 
 build_ext_tools:
   allow_failure: true
-  image: mcr.microsoft.com/dotnet/core/sdk:3.1
+  image: mcr.microsoft.com/dotnet/sdk:5.0
   needs:
     - build
   rules:
@@ -32,7 +32,7 @@ build_ext_tools:
   stage: test
 
 check_style:
-  image: mcr.microsoft.com/dotnet/core/sdk:3.1
+  image: mcr.microsoft.com/dotnet/sdk:5.0
   needs:
     - job: build
       artifacts: false
@@ -95,7 +95,7 @@ run_tests:
     reports:
       junit:
         - test_output/*.coverage.xml
-  image: mcr.microsoft.com/dotnet/core/sdk:3.1
+  image: mcr.microsoft.com/dotnet/sdk:5.0
   needs:
     - job: build
       artifacts: false

Common.props

@@ -3,7 +3,7 @@
 		<AnalysisLevel>5</AnalysisLevel>
 		<ErrorReport>prompt</ErrorReport>
 		<IsTargetingNetFramework>$(TargetFramework.StartsWith("net4"))</IsTargetingNetFramework>
-		<LangVersion>8.0</LangVersion>
+		<LangVersion>9.0</LangVersion>
 		<PlatformTarget>AnyCPU</PlatformTarget>
 		<RunAnalyzersDuringBuild Condition=" '$(MachineRunAnalyzersDuringBuild)' == '' ">false</RunAnalyzersDuringBuild>
 		<RunCodeAnalysis>false</RunCodeAnalysis>

global.json

@@ -1,6 +1,6 @@
 {
 	"sdk": {
-		"version": "3.1.100",
+		"version": "5.0.100",
 		"rollForward": "latestFeature"
 	}
 }

src/BizHawk.Tests/BizHawk.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<PropertyGroup>
-		<TargetFramework>netcoreapp3.1</TargetFramework>
+		<TargetFramework>net5.0</TargetFramework>
 	</PropertyGroup>
 	<Import Project="../MainSlnCommon.props" />
 	<Import Project="../../CommonNullable.props" />