Update ENV vars in README

What does this MR do?

Updates our documentation of ENV vars.

• Remove SAST_DEFAULT_ANALYZERS_ENABLED. It has been deprecated for 7 months, and was broken anyway: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14070/diffs#note_180248424
• Add SAST_BANDIT_EXCLUDED_PATHS. We have been supporting it, but it was not documented: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14070/diffs#note_180251270

Edit: corresponding changes now in CE! https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29564

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/10842

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
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
  • Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer