set pessimistic-txn.enabled=false in 4.x version, when upgrade to 5.0.0, the txn mode maybe changed

[seiya-annie]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. config pessimistic-txn.enabled=false in 4.x version,
2. upgrade to 5.0.0 version
3. check default txn mode

2. What did you expect to see? (Required)

current behavior maybe has the risk of incompatible, should record in release notes

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

v5.0.0-nightly

[zyguan]

Since it's a major version upgrade, and we've already announce that the config is replaced by system variable tidb_txn_mode in our release notes. I don't think we need to guarantee that compatibility.

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.