Update dependency gitlab-agent to v18.4.0

This MR contains the following updates:

Package	Update	Change
gitlab-agent	minor	v18.3.2 -> v18.4.0

MR created with the help of gitlab-org/frontend/renovate-gitlab-bot

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Release Notes

gitlab-org/cluster-integration/gitlab-agent (gitlab-agent)

v18.4.0

Compare Source

Release for tag v18.4.0

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