chore: default enalbe merge into

[BohuTANG]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

Default enable merge into.
The set enable_experimental_merge_into = 1 in the test will remove after 2 release.

• Fixes Feature Request: enable merge into by default #15033

Tests

• Unit Test(Exists Test)
• Logic Test(Exists Test)
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

[dantengsky]

cc @JackTan25 Congrats! 🎉 Your baby has finally grown up. It's been amazing to see the journey from the first steps to this moment of maturity.