Extract constants of MigrationRule

[BurningCN]

What is the purpose of the change

Extract constants of MigrationRule

[codecov-commenter]

Codecov Report

Merging #8850 (733883f) into 3.0 (1b100f8) will increase coverage by 0.04%.
The diff coverage is 41.66%.

@@             Coverage Diff              @@
##                3.0    #8850      +/-   ##
============================================
+ Coverage     63.70%   63.75%   +0.04%     
- Complexity      312      315       +3     
============================================
  Files          1145     1169      +24     
  Lines         48188    49226    +1038     
  Branches       7255     7361     +106     
============================================
+ Hits          30699    31383     +684     
- Misses        14123    14419     +296     
- Partials       3366     3424      +58     

Impacted Files	Coverage Δ
...registry/client/migration/model/MigrationRule.java	72.43% <0.00%> (ø)
...istry/client/migration/model/SubMigrationRule.java	83.72% <100.00%> (ø)
...pring/context/event/DubboBootstrapStatedEvent.java	0.00% <0.00%> (-66.67%)	⬇️
...pring/context/event/DubboBootstrapStopedEvent.java	0.00% <0.00%> (-66.67%)	⬇️
...ing/context/DubboBootstrapApplicationListener.java	0.00% <0.00%> (-60.00%)	⬇️
.../DubboBootstrapStartStopListenerSpringAdapter.java	42.85% <0.00%> (-57.15%)	⬇️
...he/dubbo/rpc/cluster/support/MergeableCluster.java	50.00% <0.00%> (-50.00%)	⬇️
...nfig/spring/extension/SpringExtensionInjector.java	56.66% <0.00%> (-30.84%)	⬇️
...ubbo/config/spring/status/SpringStatusChecker.java	55.00% <0.00%> (-29.22%)	⬇️
...ig/metadata/ServiceInstanceHostPortCustomizer.java	68.75% <0.00%> (-25.00%)	⬇️
... and 268 more

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[wangchengming666]

LGTM