Add notification when pipeline owner unavailable

What does this MR do and why?

This MR resolves an issue where pipeline_schedule was silently being skipped when the original owner is not longer a user associated with the project or has been removed entirely.

• add validation on service to check if owner still exists and member of the project
• if not then notification sent to owners and maintainers of the project

References

Screenshots or screen recordings

HTML Email

Plain Text Email

How to set up and validate locally

1. Ensure you have a pipeline schedule created in your local db and if not add one:

$ bundle exec rails console

 pry(main)> Ci::Pipeline.create(
 description: "Test Pipeline Schedule",
 ref: "1234",
 cron: "* * * * *",
 cron_timezone: "America/New_York",
 project_id: Project.first,
 owner_id: User.first
)

2. Navigate to http://gdk.test:3000/rails/mailers/notify/pipeline_schedule_owner_unavailable (or replace gdk.test with localhost name)
3. Use drop down to change format from html to plan text (or navigate to http://gdk.test:3000/rails/mailers/notify/pipeline_schedule_owner_unavailable.txt?locale=en)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #36806 (closed)