Update build ci, dockerfiles to .Net5.0

.ci/azure-pipelines-main.yml

@@ -1,7 +1,7 @@
 parameters:
   LinuxImage: 'ubuntu-latest'
   RestoreBuildProjects: 'Jellyfin.Server/Jellyfin.Server.csproj'
-  DotNetSdkVersion: 3.1.100
+  DotNetSdkVersion: 5.0.100
 
 jobs:
   - job: Build

.ci/azure-pipelines.yml

@@ -6,7 +6,7 @@ variables:
 - name: RestoreBuildProjects
   value: 'Jellyfin.Server/Jellyfin.Server.csproj'
 - name: DotNetSdkVersion
-  value: 3.1.100
+  value: 5.0.100
 
 pr:
   autoCancel: true