add prepare migration script for 2.15.0

[MinerYang]

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.92%. Comparing base (c8c11b4) to head (5e8d265).
⚠️ Report is 656 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #22788       +/-   ##
===========================================
+ Coverage   45.36%   65.92%   +20.55%     
===========================================
  Files         244     1074      +830     
  Lines       13333   116413   +103080     
  Branches     2719     2937      +218     
===========================================
+ Hits         6049    76742    +70693     
- Misses       6983    35423    +28440     
- Partials      301     4248     +3947     

Flag	Coverage Δ
unittests	65.92% <ø> (+20.55%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 989 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[wy65701436]

lgtm