Remove image.tag from kas default valuees

What does this MR do?

Removes image.tag from kas default values, given we now have !1680 (diffs)

Related issues

https://gitlab.com/gitlab-org/release-tools/-/issues/489

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Test plan

Render deployment template comparing master (a.yaml) to this branch (b.yaml)

$ diff a.yaml b.yaml 
42c42
<           image: "registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/kas:v13.7.0"
---
>           image: "registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/kas:latest"

a.yaml b.yaml

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• [-] Documentation created/updated
• [-] Tests added
• Integration tests added to GitLab QA, gitlab-org/gitlab!49053 (merged)
• Equivalent MR/issue for omnibus-gitlab opened, see gitlab-org/omnibus-gitlab!4908 (merged)