Bump strong_migrations from 0.6.6 to 0.6.8 (#13792)

Bumps [strong_migrations](https://github.com/ankane/strong_migrations) from 0.6.6 to 0.6.8.
- [Release notes](https://github.com/ankane/strong_migrations/releases)
- [Changelog](https://github.com/ankane/strong_migrations/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ankane/strong_migrations/compare/v0.6.6...v0.6.8)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Gemfile.lock

@@ -607,7 +607,7 @@ GEM
     stoplight (2.2.0)
     streamio-ffmpeg (3.0.2)
       multi_json (~> 1.8)
-    strong_migrations (0.6.6)
+    strong_migrations (0.6.8)
       activerecord (>= 5)
     temple (0.8.2)
     terminal-table (1.8.0)