Announce deprecation of bundler-audit
### Problem to solve
`bundler-audit` will be deprecated in `15.0` and this deprecation has to be announced.
### Proposal
Warn users about this deprecation in the analyzer itself and the DS documentation.
### Implementation plan
- [x] update Dependency Scanning docs with a warning about deprecation (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/76408)
- [x] update the `bundler-audit` analyzer with a warning in the job output (https://gitlab.com/gitlab-org/security-products/analyzers/bundler-audit/-/merge_requests/88)
- [x] have deprecation release post in 14.6 announcing deprecation and if users customized yml to update it and coverage is continued through gemnasium (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/76191)
- [x] Include in "SCA deprecations" overview blog post (https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/95044)
- [x] communicate to support slack channels and link to issue
issue