Update NGINX Ingress to 1.15.1

What does this MR do?

Update NGINX Ingress to 1.15.1

• Update chart version by using scripts/update-nginx-chart.sh.
• Update default image tag to 1.15.1.

Changelog: changed

Related issues

Closes Update to NGINX Ingress 1.15 (#6331 - closed)

Author checklist

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Required

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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.