Add priorityClassName to gitlab-pages pods

What does this MR do?

With this MR the gitlab-pages subchart will be able to set priorityClassName to the pods. Currently i have added a priorityClassName configuraton for my Gitlab deployment but for the gitlab-pages pods it is missing.

Related issues

Author checklist

See Definition of done.

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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
• When ready for review, follow the instructions in the "Reviewer Roulette" section of the Danger Bot MR comment, as per the Distribution experimental MR workflow

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Expected (please provide an explanation if not completing)

• Tests added/updated
• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.