docs: add issue and merge request templates

The repo had no .gitlab/ directory, so an issue or MR started from a blank box, and SECURITY.md's confidential-issue link landed a reporter on one too.

This adds four templates. Vulnerability report mirrors SECURITY.md's five reporter fields verbatim and opens with /confidential. Bug and Upstream request split an issue in this build (Dockerfiles, pipeline, packaging) from a request for kaniko's own upstream behavior, pointing at the standing fork-choice issue for that argument. Default is a contributor checklist drawn from CONTRIBUTING.md.

SECURITY.md's confidential-issue link now carries issuable_template, the query parameter GitLab's own docs name for pre-selecting an issue template, so it opens straight into Vulnerability report.

site/ and brand/ untouched. scripts/check-versions.sh passes; no version pin changed.