Fix sorting issues by "last updated" after import from GitHub

## What does this MR do?

Don't touch Issue/Merge Request when importing GitHub comments as it will trigger an update on `updated_at` field. It also use `updated_at` as the last updated date doesn't matter the Issue/Pull Request state.

## Why was this MR needed?

After import from GitHub, sorting issues by "last updated" doesn't work as expected.

## What are the relevant issue numbers?

Fixes #21567

See merge request !6110