Rename Browserker configuration options to browser

What does this MR do?

Renames the Browserker configuration options to browser inline with the direction to change the public facing name of Browserker.

What are the relevant issue numbers?

gitlab-org/gitlab#292675 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer