Remove support for gitlab_pages['use_legacy_storage'] setting

What does this MR do?

Remove support for gitlab_pages['use_legacy_storage'] setting, see #6166 (closed). The flag was both introduced deprecated in %14.0 and is scheduled for removal in %14.3.

This removal differs from our typical removals which wait for major releases, in that this flag was planned for removal here from the very beginning.

• In 14.0 we removed domain_config_source which had been previously deprecated, and allowed users to specify disk storage
• In 14.0 we added use_legacy_storage as a temporary flag to unblock upgrades, and allow us to debug issues with our customers and it was deprecated and communicated for removal in 14.3 from the very beginning.

Related issues

Closes #6166 (closed)
Related to gitlab-pages#561 (closed)

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

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 the GitLab Chart opened