Bump version to 7.5.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.4.0...ceea75db34643eb8e81180c48dbae9cebc130dda

• Release notes:

### Fixes

- !720 Update airgapped netcat flag deprecation.

### Doc changes

- !700 Document the use of WEBDRIVER_HEADLESS instead of CHROME_HEADLESS.

### Other changes (CI, backstage)

- !713 Add solargraph gem to development dependencies
- !714 Generate aggregated allure test report for all tests executed in pipeline
- !716 Allow Praefect jobs to fail while under investigation
- !718 Override unnecessary defaults for allure report generation job
- !719 Revert CI change that allowed ee:praefect to fail.

• 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.