Bump VERSION to 5.0.3

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.0.2...7a07cfee245140cff07a293fc5d8b143762c1a4b

• Release notes:

### Fixes

- !406 Use the Issues API instead of search

• 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:

  • Create a tag for the new release version.