fix: use the object heap to keep the min ddl ts order

[SimFG]

issue: #39002

[codecov]

Codecov Report

Attention: Patch coverage is 89.55224% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.17%. Comparing base (d6206ad) to head (1a8905a).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/util/typeutil/heap.go	86.04%	4 Missing and 2 partials ⚠️
internal/rootcoord/scheduler.go	94.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #39118       +/-   ##
===========================================
+ Coverage   69.64%   81.17%   +11.53%     
===========================================
  Files         296     1395     +1099     
  Lines       26633   197276   +170643     
===========================================
+ Hits        18548   160147   +141599     
- Misses       8085    31529    +23444     
- Partials        0     5600     +5600     

Components	Coverage Δ
Client	79.53% <ø> (∅)
Core	69.64% <ø> (ø)
Go	83.11% <89.55%> (∅)

Files with missing lines	Coverage Δ
internal/rootcoord/task.go	93.18% <100.00%> (ø)
internal/rootcoord/scheduler.go	94.69% <94.11%> (ø)
pkg/util/typeutil/heap.go	93.25% <86.04%> (ø)

... and 1096 files with indirect coverage changes

[chyezh]

/lgtm

[xiaofan-luan]

/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: SimFG, xiaofan-luan

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