test(spanner): update max staleness bound value

[harshachinta]

TestIntegration_SingleUse/max_staleness is showing flaky behavior and this PR fixes it.

Background: The current test has a Max staleness value of 1 sec. This Bounded staleness mode allow Spanner to pick the read timestamp, subject to a user- provided staleness bound. Spanner chooses the newest timestamp within the provided staleness bound. More details [here](https://cloud.google.com/spanner/docs/timestamp-bounds#bounded_staleness).

Flakiness Reason: The test verifies whether the chosen read timestamp is after writes[3] timestamp. But since the staleness value is 1 sec, the chosen timestamp by spanner can be much before the writes[3] timestamp which gices a flaky behaviour with below error

SingleUse.Query doesn't return expected timestamp: read got timestamp 2024-05-02 07:02:58.245414 +0000 UTC, want it to be no earlier than 2024-05-02 07:02:58.289881 +0000 UTC

Choosing a lower value of staleness ensures that the chosen timestamp is always above the writes[3] timestamp.

Fixes #10084

[rahul2393]

Can we do this, instead of reducing the max staleness, so that the check is more understandable?