Standards for hook_update_N helpers

Spin off from #17: admin: node settings displayed in both "settings and filters" an

We currently have to conventions for naming helper functions to use hook_update_N() :
- update_[N]000_*() for update-safe versions of API functions:
_update_8000_field_sql_storage_write()
_update_7000_field_create_field()
_update_7000_node_get_types()
...
- update_*() for "generic helpers"
update_variable_get|set|del() - operate on {variables}, probably bound to disappear at some point
update_variables_to_config()
update_variables_to_state()
update_add_uuids()
update_config_manifest_add() - added by #1817182: Add upgrade path tests for contact category config conversion
Some/most of them will not be relevant in D9 anymore.

The latter then sit next to other update_*() functions internal to the update.php mechanisms:
update_fix_compatibility()
update_build_dependency_graph(),
update_is_missing()

I think we should more clearly separate "helper funcs for update hooks" and "internal update.php stuff"
For instance: standardize on _update_[N]000_*() for helper functions ?