refactor(refresh): use the refresh module from the single kernel library

[marceloneppel]

Issue

Part of the refresh module migration into the single kernel library (canonical/postgresql-single-kernel-library).

Solution

The VM charm now consumes the library's RefreshManager instead of owning its own refresh plumbing:

• The charm-specific _PostgreSQLRefresh class and the refresh-only helpers (_post_snap_refresh, _reconcile_refresh_status, _check_and_update_internal_cert, _migrate_temp_tablespace_location, _execute_temp_tablespace_migration, _resolve_primary_host) are deleted — they now live in the library.
• The collect-unit-status observer targets the manager's status reconciliation, keeping the refresh status gate and the "do not use collect status events elsewhere" contract intact.
• The post-init resume block (snap-refresh flow, pre-refresh checks, temp tablespace migration) is replaced by RefreshManager.on_init().
• set_unit_status delegates to the manager, and a refresh property keeps watcher.py and async_replication.py working unchanged.
• Side effects owned by not-yet-migrated modules (exporters, pgBackRest service, watcher address) move behind a post_refresh_side_effects bridge, alongside the other duck-typed bridge methods (set_default_unit_status, set_app_status, get_async_primary_cluster_endpoint, has_async_replication_relation, update_relation_endpoints).
• The library is pinned to the archive tarball of 9b26a57eb347e6dd79a31afbdc96c319a7600697 until the next release.

Unit tests that exercised the charm-side helpers are repointed at the manager's equivalents.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.