fix: reset Migrator CURRENT_VERSION to the last released version

Hand-setting CURRENT_VERSION to the upcoming release (1.0.0) left
release-please with nothing to bump, so Migrator.php was absent from
the release PR's diff. Source files should always carry the last
released version; release-please owns the bump from N-1 to N via the
block markers.

Reset to 0.5.0 (current main's plugin.php Version). The next release
PR will bump both plugin.php and Migrator.php in lockstep, and the
mechanism continues working for every future release.

Author: nlemoine

src/Lifecycle/Migrator.php

@@ -16,7 +16,7 @@ final class Migrator
 {
     private const DB_VERSION_OPTION = 'pfcpt_db_version';
     // x-release-please-start-version
-    private const CURRENT_VERSION = '1.0.0';
+    private const CURRENT_VERSION = '0.5.0';
     // x-release-please-end
 
     public function migrate(): void