1. Update DowngradeUpgradeMembersByID
If it's downgrading process, the desire version of cluster should be
target one.
If it's upgrading process, the desire version of cluster should be
determined by mininum binary version of members.
2. Remove AssertProcessLogs from DowngradeEnable
The log message "The server is ready to downgrade" appears only when the storage
version monitor detects a mismatch between the cluster and storage versions.
If traffic is insufficient to trigger a commit or if an auto-commit occurs right
after reading the storage version, the monitor may fail to update it, leading
to errors like:
```bash
"msg":"failed to update storage version","cluster-version":"3.6.0",
"error":"cannot detect storage schema version: missing confstate information"
```
Given this, we should remove the AssertProcessLogs statement.
Similar to #19313
Signed-off-by: Wei Fu <fuweid89@gmail.com>
