refactor: exclude thumbnail directories when backup (#6725)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.20.x

#### What this PR does / why we need it:
备份时忽略缩略图目录

#### Which issue(s) this PR fixes:
Fixes #6717

#### Does this PR introduce a user-facing change?
```release-note
备份时忽略缩略图目录以减少文件大小
```

application/src/main/java/run/halo/app/migration/impl/MigrationServiceImpl.java

@@ -69,7 +69,8 @@ public class MigrationServiceImpl implements MigrationService, InitializingBean
         "mysql/**",
         "mysqlBackup/**",
         "**/.idea/**",
-        "**/.vscode/**"
+        "**/.vscode/**",
+        "attachments/thumbnails/**"
     );
 
     private final DateTimeFormatter dateTimeFormatter;