Handle securityContext for self-signed-cert-job the same way as shared secrets job

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What does this MR do?

This MR handles the securityContext for the self-signed-cert job the same way as for the shared-secrets job.

Having two different configuration approaches using the same entry in values.yaml leads to inconsistencies during deployment.

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart.