fix: resolve concurrency issue causing duplicate thumbnail generation

[guqing]

What type of PR is this?

/kind improvement
/area core
/milestone 2.20.x

What this PR does / why we need it:

修复可能为因为并发调用缩略图生成导致多次创建缩略图的问题

此 PR 为 #7031 的补充，并且会清理以前重复生成的缩略图记录和文件

Does this PR introduce a user-facing change?

修复可能为因为并发调用缩略图生成导致多次重复缩略图记录的问题

[codecov]

Codecov Report

Attention: Patch coverage is 53.44828% with 27 lines in your changes missing coverage. Please review.

Project coverage is 57.25%. Comparing base (eff73dc) to head (b4ee2ec).
Report is 64 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/halo/app/core/attachment/ThumbnailMigration.java	43.75%	27 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7077      +/-   ##
============================================
+ Coverage     56.99%   57.25%   +0.25%     
- Complexity     3999     4020      +21     
============================================
  Files           714      713       -1     
  Lines         24110    24209      +99     
  Branches       1585     1593       +8     
============================================
+ Hits          13742    13861     +119     
+ Misses         9756     9733      -23     
- Partials        612      615       +3     

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ruibaby]

/lgtm

[JohnNiang]

/approve

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang

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