Announce deprecation of bundler-audit

bundler-audit will be deprecated in 15.0 and this deprecation has to be announced.

Warn users about this deprecation in the analyzer itself and the DS documentation.

update Dependency Scanning docs with a warning about deprecation (!76408 (merged))
update the bundler-audit analyzer with a warning in the job output (gitlab-org/security-products/analyzers/bundler-audit!88 (merged))
have deprecation release post in 14.6 announcing deprecation and if users customized yml to update it and coverage is continued through gemnasium (!76191 (merged))
Include in "SCA deprecations" overview blog post (gitlab-com/www-gitlab-com!95044 (merged))
communicate to support slack channels and link to issue

Edited Feb 08, 2022 by Katrin Leinweber