Project: update `updated_at` if other timestamps are changed

What does this MR do and why?

In the Project model, there are the two timestamps last_activity_at and last_repository_updated_at. These are currently updated without the timestamp updated_at being updated as well. So we have three timestamps, which all hold the last change timestamp of a part of the project. To get the timestamp of the actual last change, the maximum of the three timestamps must be calculated. If you want to sort a project list by the timestamp of the actual last change, you will encounter database performance problems.

We tried to solve this problem with different approaches:

• (A) Add an index using GREATEST(updated_at, last_activity_at, last_repository_updated_at) - see !75350 (closed)
• (B) Add a separate column containing the actual greatest timestamp using a trigger - see !77497 (closed)

Unfortunately, with (A) the existing datetime inconsistencies lead to difficulties and (B) is not an ideal solution from a database point of view. So the question arose, why the updated_at column is actually not automatically updated by changing last_activity_at or last_repository_updated_at and thus always contains the actual timestamp of the last change. And according to !77497 (comment 814805839), there's no reason against it.

So this MR ensures that when last_activity_at or last_repository_updated_at is changed, updated_at is updated as well.

🛠 with ❤ at Siemens

/cc @bufferoverflow

How to set up and validate locally

1. Navigate to a project in the UI
2. Change a project setting (e.g. description) or star/unstar the project ➡ updated_at is updated
3. Create an issue or milestone ➡ last_activity_at and thus updated_at are updated
4. Push to the repository ➡ last_repository_updated_at and thus updated_at are updated

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