[PM-5052] Upgrade to .NET 8 (#3461)

* Upgrade to .NET 8

* Linting

* Clean up old JSON deserialization code

* More .NET 8-oriented linting

* Light feedback

* Get rid of old test we don't know the root issue for

* Fix a new test

* Remove now-unnecessary Renovate constraint

* Use Any()

* Somehow a 6.0 tooling config we don't need snuck back in

* Space out properties that always change per release

* Bump a few core packages since the last update

src/Core/AdminConsole/OrganizationFeatures/OrganizationDomains/CreateOrganizationDomainCommand.cs

@@ -59,7 +59,7 @@ public class CreateOrganizationDomainCommand : ICreateOrganizationDomainCommand
         }
         catch (Exception e)
         {
-            _logger.LogError("Error verifying Organization domain.", e);
+            _logger.LogError(e, "Error verifying Organization domain.");
         }
 
         organizationDomain.SetNextRunDate(_globalSettings.DomainVerification.VerificationInterval);

src/Core/OrganizationFeatures/OrganizationSponsorships/FamiliesForEnterprise/Cloud/ValidateSponsorshipCommand.cs

@@ -88,7 +88,7 @@ public class ValidateSponsorshipCommand : CancelSponsorshipCommand, IValidateSpo
             }
             catch (Exception e)
             {
-                _logger.LogError("Error sending Family sponsorship removed email.", e);
+                _logger.LogError(e, "Error sending Family sponsorship removed email.");
             }
         }
         await base.DeleteSponsorshipAsync(sponsorship);

src/Core/Utilities/JsonHelpers.cs

@@ -45,18 +45,6 @@ public static class JsonHelpers
         };
     }
 
-    [Obsolete("This is built into .NET 6, it SHOULD be removed when we upgrade")]
-    public static T ToObject<T>(this JsonElement element, JsonSerializerOptions options = null)
-    {
-        return JsonSerializer.Deserialize<T>(element.GetRawText(), options ?? Default);
-    }
-
-    [Obsolete("This is built into .NET 6, it SHOULD be removed when we upgrade")]
-    public static T ToObject<T>(this JsonDocument document, JsonSerializerOptions options = null)
-    {
-        return JsonSerializer.Deserialize<T>(document.RootElement.GetRawText(), options ?? default);
-    }
-
     public static T DeserializeOrNew<T>(string json, JsonSerializerOptions options = null)
         where T : new()
     {

src/Core/Utilities/SecurityHeadersMiddleware.cs

@@ -15,13 +15,13 @@ public sealed class SecurityHeadersMiddleware
     public Task Invoke(HttpContext context)
     {
         // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options
-        context.Response.Headers.Add("x-frame-options", new StringValues("SAMEORIGIN"));
+        context.Response.Headers.Append("x-frame-options", new StringValues("SAMEORIGIN"));
 
         // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection
-        context.Response.Headers.Add("x-xss-protection", new StringValues("1; mode=block"));
+        context.Response.Headers.Append("x-xss-protection", new StringValues("1; mode=block"));
 
         // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options
-        context.Response.Headers.Add("x-content-type-options", new StringValues("nosniff"));
+        context.Response.Headers.Append("x-content-type-options", new StringValues("nosniff"));
 
         return _next(context);
     }

src/Core/Utilities/SpanExtensions.cs

@@ -18,21 +18,4 @@ public static class SpanExtensions
         rest = input[++splitIndex..];
         return true;
     }
-
-    // Replace with the implementation from .NET 8 when we upgrade
-    // Ref: https://github.com/dotnet/runtime/issues/59466
-    public static int Count<T>(this ReadOnlySpan<T> span, T value)
-        where T : IEquatable<T>
-    {
-        var count = 0;
-        int pos;
-
-        while ((pos = span.IndexOf(value)) >= 0)
-        {
-            span = span[++pos..];
-            count++;
-        }
-
-        return count;
-    }
 }