Update runner version only when updating contacted_at

## What does this MR do?

Improves how we update runners table, especially the version.

This is another round of improvements to reduce number of `ci_runners` updates.

I did make `contacted_at` to be updated more often (on average every 15 minutes).
We will also update version information in one go to solve: https://gitlab.com/gitlab-org/gitlab-ce/issues/22206

Improves: https://gitlab.com/gitlab-org/gitlab-ce/issues/22590
Solves: https://gitlab.com/gitlab-org/gitlab-ce/issues/22206

See merge request !6537