Merge branch 'jv-multipart-no-boundary' into 'master'

Return HTTP 400 for multipart header without boundary

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/106420



Merged-by: Alessio Caiazza <acaiazza@gitlab.com>
Approved-by: Ash McKenzie <amckenzie@gitlab.com>
Approved-by: Alessio Caiazza <acaiazza@gitlab.com>
Reviewed-by: Ash McKenzie <amckenzie@gitlab.com>
Co-authored-by: Jacob Vosmaer <jacob@gitlab.com>