Allow shell maxUnavailable to be a percentage

What does this MR do?

Fixes maxUnavailable type in JSON schema for gitlab-shell, it can be a number or string (percentage based).

gitlab:
  gitlab-shell:
    enabled: true
    maxUnavailable: 5%

  Error: Failed to render chart: exit status 1: Error: values don't meet the specifications of the schema(s) in the following chart(s):
  gitlab-shell:
  - maxUnavailable: Invalid type. Expected: number, given: string

Related issues

gitlab-com/gl-infra/k8s-workloads/gitlab-com!1873 (merged)

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

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
• Equivalent MR/issue for omnibus-gitlab opened