Update KAS to 15.0.0 in pre and gstg

What does this MR do?

Updates KAS to 15.0.0 in pre and gstg.

Testing

docker run --rm -it dev.gitlab.org:5005/gitlab/charts/components/images/gitlab-kas:15-0-202205122020-9594a5eea5e --version

outputs

kas version v15.0.0, commit: b93d7a6, built: 20220512.203636

Author Check-list

Please read the Contributing document and once you do, complete the following:

• Assign to the correct reviewer per the contributing document
• Apply the correct metadata per the contributing document
• Link to related MRs for applying the changes on other environments
• Link to related Chef changes
• If necessary link to a Criticality 4 Change Request issue

Reviewer Check-list

• Reviewed the diff jobs to confirm changes are as expected
• No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation

Applier Check-list

• Make sure there is no ongoing deployment for the affected envs before merging (see #announcements slack channel)