GitHubCI: upgrade .NET to fix restore issue

This new version doesn't have the restore bug[1] we were
hitting, per this comment[2]:

```
Jellyfin.Plugins.Telegram.csproj : error NU3028: Package 'System.Threading.Tasks.Extensions 4.3.0' from source
'https://api.nuget.org/v3/index.json': The repository primary signature's timestamp found a chain building issue: UntrustedRoot: self signed certificate in certificate chain
```

[1] dotnet/core#6595
[2] dotnet/core#6595 (comment)

Author: knocte

.github/workflows/main.yml

@@ -7,7 +7,7 @@ on:
   workflow_dispatch:
 
 env:
-  DOTNET_VERSION: 5.0.102
+  DOTNET_VERSION: 5.0.302
 
 jobs:
   build: