[Improve] Rename getCountSql to getExistDataSql

[Hisoka-X]

Purpose of this pull request

This PR rename method getCountSql to getExistDataSql in AbstractJdbcCatalog. Becuase it used to check data exist or not in database, not used to get data count.

Does this PR introduce any user-facing change?

no

How was this patch tested?

exist test.

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If you are contributing the connector code, please check that the following files are updated:
  1. Update change log that in connector document. For more details you can refer to connector-v2
  2. Update plugin-mapping.properties and add new connector information in it
  3. Update the pom file of seatunnel-dist
• Update the release-note.

[Carl-Zhou-CN]

+1