Investigate Import feature flags

Summary

We need to investigate whether any of the feature flags in Import should be enabled by default. This issue discusses the reasons behind the ask.

List of feature flags for Import

name: remove_legacy_github_client
introduced_by_url: 
rollout_issue_url: 
milestone: 
type: development
group: group::import
default_enabled: false

name: bitbucket_server_user_mapping_by_username
introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36885
rollout_issue_url: 
milestone: '13.4'
type: development
group: group::import
default_enabled: false

name: export_reduce_relation_batch_size
introduced_by_url: 
rollout_issue_url: 
milestone: 
type: development
group: group::import
default_enabled: false

name: bulk_import
introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42704
rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/255310
milestone: '13.5'
type: development
group: group::import
default_enabled: false

name: validate_import_decompressed_archive_size
introduced_by_url: 
rollout_issue_url: 
milestone: 
type: development
group: group::import
default_enabled: false

name: log_import_export_relation_creation
introduced_by_url: 
rollout_issue_url: 
milestone: 
type: development
group: group::import
default_enabled: false

name: phabricator_import
introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/13569
rollout_issue_url: https://gitlab.com/groups/gitlab-org/-/epics/1197
milestone: '12.0'
type: development
group: group::import
default_enabled: false