Release 8.17.3

Picked into respective stable branches:
- merge requestsPick into Stable 8.17 merged merge requests
Merge ce/8-17-stable into ee/8-17-stable-ee
Check omnibus-gitlab/8-17-stable CHANGELOG.md
Check omnibus-gitlab/8-17-stable-ee CHANGELOG.md
While waiting for tests to be green, now is a good time to start on the blog post: gitlab-com/www-gitlab-com!5264 (merged)
Ensure tests are green on CE
Ensure tests are green on EE
Check for any problematic migrations in EE (EE migrations include CE ones), and paste the diff in a snippet: git diff v8.17.2-ee..8-17-stable-ee -- db/migrate => 0 migrations

Tag the 8.17.3 version using the release task:

```sh
bundle exec rake "release[8.17.3]"
```

Check that EE packages are built, CE packages are built and appears on packages.gitlab.com: EE / CE

In #infrastructure:

```
I'm going to deploy `8.17.3` to staging
```

In #infrastructure:

```
I'm going to deploy `8.17.3` to production
```

```
GitLab 8.17.3 is released! BLOG_POST_URL DESCRIPTION OF THE CHANGES
```

In the 8.17 Regressions issue:
- Add the following notice:
```
`8.17.3` has been tagged, further fixes will go into `8.17.4` as necessary.
```
- Remove notes for the regressions fixed by version 8.17.3
Add omnibus-gitlab/8.17.3+ce.0 CHANGELOG.md items to omnibus-gitlab/master CHANGELOG.md

For references: