planner: enable cost model2 by default

[qw4990]

What problem does this PR solve?

Issue Number: close #35240

Problem Summary: planner: enable cost model2 by default

What is changed and how it works?

planner: enable cost model2 by default

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• fzzf678

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[qw4990]

/run-unit-test

[qw4990]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: fb087a9

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [2] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci/integration-cdc-test	🟥 failed 2, success 37, total 39	20 min	New failing
idc-jenkins-ci-tidb/sqllogic-test-1	🟥 failed 1, success 25, total 26	4 min 16 sec	New failing
idc-jenkins-ci-tidb/mybatis-test	🔴 failed 1, success 0, total 1	11 min	Existing failure
idc-jenkins-ci-tidb/integration-common-test	✅ all 17 tests passed	13 min	Fixed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	10 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 48 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	5 min 4 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	4 min 55 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 33 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed