feat: enable dynamic mapping when migrate

[SuZhou-Joe]

Description

try to make workspaces field dynamic because workspace is a experimental feature.

Issues Resolved

Screenshot

Testing the changes

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
  • yarn test:ftr
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 55.45%. Comparing base (587f0d4) to head (f9b3523).
Report is 1 commits behind head on workspace.

Files	Patch %	Lines
...d_objects/migrations/core/build_active_mappings.ts	0.00%	3 Missing ⚠️
...saved_objects/migrations/core/migration_context.ts	0.00%	2 Missing ⚠️
...earch_dashboards/opensearch_dashboards_migrator.ts	0.00%	2 Missing ⚠️
...core/server/saved_objects/saved_objects_service.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           workspace     #262       +/-   ##
==============================================
- Coverage      66.71%   55.45%   -11.26%     
==============================================
  Files           3336     3049      -287     
  Lines          64764    60812     -3952     
  Branches       10439     9836      -603     
==============================================
- Hits           43206    33726     -9480     
- Misses         18977    25027     +6050     
+ Partials        2581     2059      -522     

Flag	Coverage Δ
Linux_1	31.41% <0.00%> (+0.34%)	⬆️
Linux_2	?
Linux_3	41.74% <0.00%> (+0.03%)	⬆️
Linux_4	34.88% <0.00%> (+0.03%)	⬆️

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[Hailong-am]

this import seems not correct, might need double check