dashboard/app: add logging to legacy tags conversion

[tarasmadan]

Add INFO level logging when loading bugs that still use the legacy Tags schema.

This will help detect if the legacy conversion code is still needed in production.

[gemini-code-assist]

Code Review

This pull request imports the "log" package and adds an informational log statement in the Load method of the Bug struct in dashboard/app/entities_datastore.go to log legacy tags when loading a bug. There are no review comments, so I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.0%. Comparing base (b9f1cea) to head (ee4f2d6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

Files with missing lines	Coverage Δ
dashboard/app/entities_datastore.go	76.5% <100.0%> (+0.1%)	⬆️

... and 5 files with indirect coverage changes

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[a-nogikh]

See the comment at #7559 (comment)