ticdc: Refactor CheckIsTiDB function to remove the returned error

[wlwilliamx]

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Description:
In the CheckIsTiDB function, the current implementation returns an error which is no need for the external callers. In earlier versions, this function returned an error along with a boolean value, allowing callers to differentiate between network-related issues and the absence of TiDB. However, since the specific error content wasn't critical to the logic—external callers only needed to know whether the downstream was TiDB—the decision was made to simplify the function. External callers should not handle the error returned here because even if the downstream is not TiDB, TiCDC should still function properly. For more details, refer to #11214.

The primary goal of this issue is to refactor the function to remove the error return type. Instead of returning an error, we will log a warning if an issue occurs while querying the TiDB version, as the error content itself is not critical to the logic. This change will simplify the function while maintaining awareness of potential network issues through logging.

Action Items:

1. Remove the error return value from CheckIsTiDB.
2. Log a warning instead of an error when row.Scan() fails.
3. Add comments to clarify the logic and reason for not returning errors.