Bump version to 8.15.2

Diff

https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.15.1...4803cc6ef4ec0a4ddab706d499d6c20659558f79

Release notes:

Fixes

• Pass allow seperate ci db option to omnibus con... (!1120 - merged)

Other changes (CI, backstage)

• Change customer_portal_url to env variable (!1112 - merged)
• Ensure Gitaly & Praefect images can access priv... (!1115 - merged)
• Set GITLAB_QA_USER_AGENT as a Rails env var (!1118 - merged)
• Remove unused upstream notification code (!1119 - merged)

Checklist before merging:

• Diff link is up-to-date.
• Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
• Release notes are accurate.

Checklist after merging:

• Update the release notes for the newly created tag.