Fix DatabricksSqlHook deprecation warning on every connection

[sohailiqrar]

databricks-sql-connector deprecated the _user_agent_entry keyword argument in favour of user_agent_entry (no leading underscore). Passing the old name logs a warning on every call to get_conn(), which fires for every DatabricksSqlOperator / sensor execution. Switch to the current parameter name so the warning is no longer emitted.

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[sohailiqrar]

DatabricksSqlHook.get_conn() was passing _user_agent_entry (with a leading underscore) to sql.connect(). The databricks-sql-connector deprecated that alias and logs a [WARN] Parameter '_user_agent_entry' is deprecated warning on every connection open, which means every DatabricksSqlOperator / sensor execution produces noisy log spam.

Switch to the current parameter name user_agent_entry. No behavior change: user_agent_entry has been supported by databricks-sql-connector since well before the provider's >=4.4.0 lower bound.

closes: #72102