Upgrade Notifications Email to dotnet v9

[hggutvik]

Description

We should upgrade the Notifications Email application to .net9. .net9 has standard time support until May 12, 2026, but we should take the time to upgrade and not wait for .net 10.

Additional Information

.NET Core Support lifecycle
Container Patching

Tasks

• Update base images in the dockerfile for .net9
• Bump all projects; src and test, to .net9
• Update any build pipelines to support .net9
• Update any github workflows with .net9
• Check if we need to change anything in SonarCloud

Acceptance Criterias

• Notifications Email is built with .net9 tooling and run on .net9 Description

[Ahmed-Ghanam]

The release pipelines for this application utilize agents from the "altinn-vmss-platformagent" pool. The Platform team will update the base image used by these agents to the latest version