Add pages multiple version toggle

What does this MR do and why?

Create a kill-switch for the multi-version feature, so if someone wants to keep using pages as a single-version, without any concerns of multiple versions being introduced by other contributors, they would be able to configure that.

This feature is GitLab Premium.

Related to: #416487 (closed)

Migration

bundle exec rails db:migrate:up:main VERSION=20230811144601

main: == [advisory_lock_connection] object_id: 225460, pg_backend_pid: 35806
main: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: migrating =
main: -- add_column(:project_settings, :pages_multiple_versions_enabled, :boolean, {:default=>false, :null=>false})
main:    -> 0.0080s
main: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: migrated (0.0139s)

main: == [advisory_lock_connection] object_id: 225460, pg_backend_pid: 35806

bundle exec rails db:migrate:up:ci VERSION=20230811144601

ci: == [advisory_lock_connection] object_id: 225540, pg_backend_pid: 41174
ci: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: migrating =
ci: -- add_column(:project_settings, :pages_multiple_versions_enabled, :boolean, {:default=>false, :null=>false})
ci:    -> 0.0059s
ci: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: migrated (0.0214s)

ci: == [advisory_lock_connection] object_id: 225540, pg_backend_pid: 41174

bundle exec rails db:migrate:down:main VERSION=20230811144601

main: == [advisory_lock_connection] object_id: 225540, pg_backend_pid: 55998
main: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: reverting =
main: -- remove_column(:project_settings, :pages_multiple_versions_enabled, :boolean, {:default=>false, :null=>false})
main:    -> 0.0034s
main: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: reverted (0.0117s)

main: == [advisory_lock_connection] object_id: 225540, pg_backend_pid: 55998

bundle exec rails db:migrate:down:ci VERSION=20230811144601

ci: == [advisory_lock_connection] object_id: 225480, pg_backend_pid: 60661
ci: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: reverting =
ci: -- remove_column(:project_settings, :pages_multiple_versions_enabled, :boolean, {:default=>false, :null=>false})
ci:    -> 0.0021s
ci: == 20230811144601 AddPagesMultipleVersionsEnabledToProjectSettings: reverted (0.0130s)

ci: == [advisory_lock_connection] object_id: 225480, pg_backend_pid: 60661

Screenshots or screen recordings

How to set up and validate locally

• You need an Premium license
• Enable the pages_multiple_versions feature flag
• Run the migrations
• Create a project with Pages enabled
• Navigate to the pages settings: example.com/namespace/project/pages
• You should be able to see the Multiple Versions toggle

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.